feat(plex): Collection info page — hero, play/shuffle, and write-through editing (B)

Clicking a collection tile now opens a dedicated Collection info page (like the Plex-web
one) instead of just applying it as a filter. New GET /collections/{rk} returns the hero
aggregates (year range, most-restrictive member cert, total runtime) + the members ordered
by release year then title; a new PlexCollectionView renders the hero (art/title/meta/
summary+More) and the member grid, with a Play-all and Shuffle over the playable leaves
(reusing the player's existing queue + auto-advance).

For a non-external, admin-enabled collection the page offers rename / delete / add-title
(a search picker) / remove-item, all through the pre-existing admin endpoints that write
THROUGH to Plex (each edit awaits and re-fetches — no optimism, since a Plex write can 422).
An eligible collection shows "Enable editing" first (the DB editable flag). Read-only ones
(smart / imdb / tmdb / … auto-lists) are marked with a lock on both the info page AND the
browse tile, up front.

New `{ kind: "collection" }` sub-view wired through plexSubUrl (bookmarkable ?v=collection:rk),
PlexBrowse (render branch, Backspace back-nav, tile handler), with a round-trip test.

Verified live: tile→page, hero + chronological members (James Bond 1962→2021, PG-13, 55h),
Play/Shuffle, Enable editing → edit controls + remove-X, the add search (marks already-in
titles "Added"), and the read-only lock on external collections. Real Plex mutations wired
but not executed against the live account.

Gate: tsc/lint/knip/prettier clean, 424 vitest; backend ruff clean, 6 collections tests
(+ a detail-hero/ordering assertion).

backend/app/routes/plex/collections.py

@@ -14,12 +14,29 @@ from app.models import (
     PlexShow,
     User,
 )
+from app.plex import rollup
 from app.plex import sync as plex_sync
+from app.plex.cards import movie_card, show_card
 from app.plex.client import PlexClient, PlexError, PlexNotConfigured
 from app.plex.lookups import by_rating_key
 from app.plex.query import like_escape, runtime_by_key, top_art_keys
 from app.routes.plex._shared import _content_libs
 
+# The most-restrictive → least ordering used to pick a collection's effective content rating from its
+# members (Plex shows one cert on the collection; we don't store it, so derive it).
+_CERT_RANK = {
+    "G": 1, "TV-Y": 1, "TV-Y7": 1, "TV-G": 1,
+    "PG": 2, "TV-PG": 2,
+    "PG-13": 3, "TV-14": 3,
+    "R": 4, "TV-MA": 4,
+    "NC-17": 5,
+}
+
+
+def _collection_cert(certs: list[str | None]) -> str | None:
+    ranked = [(c, _CERT_RANK.get(c, 0)) for c in certs if c]
+    return max(ranked, key=lambda x: x[1])[0] if ranked else None
+
 router = APIRouter(prefix="/api/plex", tags=["plex"])
 
 _STRIP_PROVIDERS = ("imdb", "tmdb", "tvdb", "trakt")
@@ -122,6 +139,50 @@ def collections(
     }
 
 
+@router.get("/collections/{rating_key}")
+def collection_detail(
+    rating_key: str,
+    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."""
+    col = by_rating_key(db, PlexCollection, rating_key)
+    if col is None:
+        raise HTTPException(status_code=404, detail="Unknown collection")
+    movies = db.execute(
+        select(PlexItem)
+        .where(PlexItem.kind == "movie", PlexItem.collection_keys.contains([rating_key]))
+        .order_by(PlexItem.year.asc().nullslast(), func.lower(PlexItem.title))
+    ).scalars().all()
+    shows = db.execute(
+        select(PlexShow)
+        .where(PlexShow.collection_keys.contains([rating_key]))
+        .order_by(PlexShow.year.asc().nullslast(), func.lower(PlexShow.title))
+    ).scalars().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
+    ]
+    tagged.sort(key=lambda t: (t[0] is None, t[0] or 0, t[1]))
+    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
+    ]
+    years = [y for y, _, _, _ in tagged if y is not None]
+    certs = [m.content_rating for m in movies] + [s.content_rating for s in shows]
+    return {
+        **_collection_card(col),
+        "content_rating": _collection_cert(certs),
+        "year_min": min(years) if years else None,
+        "year_max": max(years) if years else None,
+        "total_runtime_s": sum(m.duration_s or 0 for m in movies),
+        "items": items,
+    }
+
+
 def _editable_collection_or_403(db: Session, rating_key: str) -> PlexCollection:
     col = by_rating_key(db, PlexCollection, rating_key)
     if col is None: