feat(plex): the collection page sorts, and a linked collection has a name

Two halves of the same report ("Collectionben nem működik a rendezés?"). The pasted browse URL
sorts correctly — that part was working. What did not:

The Collection INFO page had no sort control at all and a hardcoded release-year-ascending order.
It now shows the browse grid's Sort control, with the browse grid's vocabulary, because a second
sort vocabulary on a neighbouring surface is exactly the kind of drift this repo keeps paying for:
`PLEX_SORT_KEYS` is now one tuple in `plex/query.py`, and a test pins BOTH implementations to it —
browse's key→column map (extracted from the handler to be reachable) and the collection page's
key→member map. A key added to the vocabulary without teaching one of them fails the gate instead
of putting a dead option in the menu.

Ordering a collection cannot be a single ORDER BY: its two legs are different tables, so the merged
member list is sorted in Python. Members with no value for the key sink to the END in both
directions (a movie with no rating is not "the worst rated" — the browse grid's `nullslast()`), and
ties always resolve title-ascending, so flipping the direction reverses the ranking rather than the
alphabet inside it. `lastwatched` needed the show leg's `resumed_at`, so the page now takes ONE
rollup pass that yields both the badge status and the recency instead of the status alone — without
it that menu entry would have ordered every show as never-watched. The default stays release-year
ascending: a franchise reads in release order, and that is what the page did before.

The other half: a shared Plex link carries the collection filter as bare rating_keys, so a link
opened cold had no name for them — the sidebar token rendered `17045` and the chip could only say
"Collection". `GET /api/plex/collections` takes a `keys=` CSV now, and a display-only hook resolves
just the missing ids. It deliberately does NOT write them into `collectionTitles`: that map is
hashed into the Plex query keys, so storing a resolved label there would split the cache from the
identical state and refetch the grid for a piece of text.

Verified in the browser on localdev against the real Plex library: the movie collection's menu
offers Length and the show collection's does not (the same scope rule as browse); rating-ascending
reorders 8.1/7.4/7.2/6.6/6.5 correctly; `lastwatched` on a show collection brings the in-progress
shows up; the choice survives a reload (it is a server pref, on its own key so opening a collection
never re-sorts the library behind it); and the reported URL now reads "Pirates of the Caribbean
Collection" in both the sidebar and the chip.

12 pytest + 8 vitest, mutation-checked against 14 mutations (direction ignored, nulls first, no
stable tiebreak, unknown key untouched, wrong default, show recency dropped, empty `keys=` falling
through to everything, a key missing from either sort map, every id refetched, ids unsorted,
fetched titles unmerged, the chip ignoring them, the hook's result never reaching the builder) —
each red on exactly the test that names it.

backend/app/routes/plex/collections.py

@@ -1,5 +1,9 @@
 """Plex collections: browse mirrored collections + the admin manual-collection editor."""
 
+from collections.abc import Callable
+from datetime import date, datetime
+from typing import Any, NamedTuple
+
 from fastapi import APIRouter, Depends, HTTPException
 from pydantic import BaseModel
 from sqlalchemy import func, select
@@ -12,6 +16,7 @@ from app.models import (
     PlexItem,
     PlexLibrary,
     PlexShow,
+    PlexState,
     User,
 )
 from app.plex import rollup
@@ -83,13 +88,20 @@ def _collection_card(c: PlexCollection) -> dict:
 def collections(
     library: str | None = None,
     q: str | None = None,
+    keys: str | None = None,
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
     """Mirrored collections as cards (most-populated first), optionally name-filtered by `q`. With a
     `library` plex_key → that one library's collections (used by the collection editor). WITHOUT one →
     the UNION across all enabled libraries (the unified-scope sidebar picker, which has no single
-    library selected)."""
+    library selected).
+
+    `keys` (CSV of rating_keys) narrows to just those. It exists because a Plex link carries the
+    collection FILTER as bare rating_keys — nothing in the URL says what "17045" is called — so the
+    sidebar token and the applied-filter chip had nothing to render but the raw id. Resolving them
+    through this endpoint costs one small row each; pulling the whole list to find two titles is
+    ~200 KB."""
     if library is not None:
         lib = (
             db.execute(select(PlexLibrary).where(PlexLibrary.plex_key == str(library)))
@@ -113,6 +125,13 @@ def collections(
     term = (q or "").strip()
     if term:
         query = query.where(PlexCollection.title.ilike(f"%{like_escape(term)}%"))
+    if keys is not None:
+        wanted = [k for k in (keys or "").split(",") if k]
+        # An explicit but empty `keys=` asks for nothing — answer nothing, rather than falling through
+        # to "every collection" (which is what an unguarded empty IN () would have to become).
+        if not wanted:
+            return {"collections": []}
+        query = query.where(PlexCollection.rating_key.in_(wanted))
     rows = (
         db.execute(
             query.order_by(
@@ -149,14 +168,69 @@ def collections(
     }
 
 
+class Member(NamedTuple):
+    """One collection member flattened to everything the Sort control can order by. The page's two
+    legs are different tables (movies and shows), so there is no single query to ORDER BY — the
+    ordering is a pure list sort over these, which is also what makes it unit-testable."""
+
+    kind: str  # "movie" | "show"
+    obj: Any  # PlexItem | PlexShow, carried through to the card serializer
+    title: str  # lower-cased; never NULL (the column is not nullable)
+    year: int | None
+    rating: float | None
+    added: datetime | None
+    release: date | None
+    duration: int | None  # movies only — a show has no single runtime
+    last_watched: datetime | None
+
+
+# Sort key -> what to read off a Member. Pinned to `PLEX_SORT_KEYS` by a test: the collection page
+# shows the SAME Sort control as the browse grid, so a key this map is missing would be an option
+# that silently does nothing here.
+MEMBER_SORT_KEYS: dict[str, Callable[[Member], Any]] = {
+    "added": lambda m: m.added,
+    "lastwatched": lambda m: m.last_watched,
+    "release": lambda m: m.release,
+    "year": lambda m: m.year,
+    "rating": lambda m: m.rating,
+    "duration": lambda m: m.duration,
+    "title": lambda m: m.title,
+}
+
+# What the page ordered by before it had a control, and still its default: a film series wants to be
+# read in release order, not by whenever Plex happened to ingest it.
+DEFAULT_MEMBER_SORT = "year"
+
+
+def sort_members(members: list[Member], sort: str, sort_dir: str) -> list[Member]:
+    """Order a collection's members by one of `PLEX_SORT_KEYS`.
+
+    Two properties worth stating because they are easy to get wrong with a plain `reverse=True`:
+    members with no value for the key sink to the END in BOTH directions (matching the browse grid's
+    `nullslast()` — a movie with no rating is not "the worst rated"), and ties always resolve
+    title-ascending, so flipping the direction reverses the ranking rather than the alphabet inside
+    it. An unknown key falls back to the page's default instead of returning an arbitrary order."""
+    key = MEMBER_SORT_KEYS.get(sort, MEMBER_SORT_KEYS[DEFAULT_MEMBER_SORT])
+    ordered = sorted(members, key=lambda m: m.title)  # stable secondary, applied first
+    have = [m for m in ordered if key(m) is not None]
+    missing = [m for m in ordered if key(m) is None]
+    have.sort(key=key, reverse=sort_dir != "asc")
+    return have + missing
+
+
 @router.get("/collections/{rating_key}")
 def collection_detail(
     rating_key: str,
+    sort: str = DEFAULT_MEMBER_SORT,
+    sort_dir: str = "asc",
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    """One collection's hero data + its members (movies + shows), ordered by release year then title —
-    the Collection info page. Read-only here; edits go through the admin mutation endpoints below."""
+    """One collection's hero data + its members (movies + shows) in the requested order — the
+    Collection info page. The sort vocabulary is the browse grid's (`PLEX_SORT_KEYS`) so the same Sort
+    control means the same thing on both surfaces; the default is release year ascending, which is
+    what this page did before it had a control. Read-only here; edits go through the admin mutation
+    endpoints below."""
     col = by_rating_key(db, PlexCollection, rating_key)
     if col is None:
         raise HTTPException(status_code=404, detail="Unknown collection")
@@ -179,19 +253,63 @@ def collection_detail(
         .all()
     )
     mstate = rollup.item_state_map(db, user.id, [m.id for m in movies])
-    sstate = rollup.state_map(db, user.id, [s.id for s in shows])
-    # Merge the two type-homogeneous lists into ONE member list ordered by (year, title).
-    tagged = [(m.year, (m.title or "").lower(), "movie", m) for m in movies] + [
-        (s.year, (s.title or "").lower(), "show", s) for s in shows
+    # ONE rollup pass over the show members, for both the aggregate badge status and `resumed_at` —
+    # the browse grid's "last watched" for a show (the newest resume/watched across its episodes). The
+    # cheaper `rollup.state_map` gives only the status, which would leave `lastwatched` unorderable on
+    # a show collection while the control still offered it.
+    show_rows = (
+        db.execute(
+            rollup.agg_subquery(db, user.id, include_resumed=True).where(
+                PlexItem.show_id.in_([s.id for s in shows])
+            )
+        ).all()
+        if shows
+        else []
+    )
+    sstate = {r.show_id: rollup.show_status(r.total, r.watched, r.inprog) for r in show_rows}
+    sresumed = {r.show_id: r.resumed_at for r in show_rows}
+
+    def _movie_last_watched(st: PlexState | None) -> datetime | None:
+        # Same definition as the browse grid's: when it was last resumed, else when it was marked
+        # watched (coalesce(progress_updated_at, watched_at)).
+        return (st.progress_updated_at or st.watched_at) if st else None
+
+    # Merge the two type-homogeneous lists into ONE member list, then order it.
+    members = [
+        Member(
+            kind="movie",
+            obj=m,
+            title=(m.title or "").lower(),
+            year=m.year,
+            rating=m.rating,
+            added=m.added_at,
+            release=m.originally_available_at,
+            duration=m.duration_s,
+            last_watched=_movie_last_watched(mstate.get(m.id)),
+        )
+        for m in movies
+    ] + [
+        Member(
+            kind="show",
+            obj=sh,
+            title=(sh.title or "").lower(),
+            year=sh.year,
+            rating=sh.rating,
+            added=sh.added_at,
+            release=sh.originally_available_at,
+            duration=None,
+            last_watched=sresumed.get(sh.id),
+        )
+        for sh in shows
     ]
-    tagged.sort(key=lambda t: (t[0] is None, t[0] or 0, t[1]))
+    members = sort_members(members, sort, sort_dir)
     items = [
-        movie_card(o, mstate.get(o.id))
-        if kind == "movie"
-        else show_card(o, sstate.get(o.id, "new"))
-        for _, _, kind, o in tagged
+        movie_card(m.obj, mstate.get(m.obj.id))
+        if m.kind == "movie"
+        else show_card(m.obj, sstate.get(m.obj.id, "new"))
+        for m in members
     ]
-    years = [y for y, _, _, _ in tagged if y is not None]
+    years = [m.year for m in members if m.year is not None]
     certs = [m.content_rating for m in movies] + [s.content_rating for s in shows]
     return {
         **_collection_card(col),