feat(plex): every card rail in the reader's language; an episode in a poster rail stands as its show (E30 S1b)

The grid served Hungarian titles, but every other card surface (Discover rails, the
show page's episodes, search, collections, the person page, playlists, Home's resume
rail, hero and comfort shelf) served English. The card serializers now take a
required keyword-only lang (a forgotten call site fails instead of going English), and
each of those reads takes ?lang=hu, with the language in its query key. A Hungarian
search finds an episode by its Hungarian title too.

An episode on a poster rail showed its 16:9 still cropped into a poster, titled only
with the episode name; it now shows the show's poster and name, with S·E · title under
it (option A). Plex's generic season names (Season 1, Specials) are rebuilt from the
number in the reader's language; a season with a name of its own keeps it.

Home's comfort-shelf move wrote its optimistic order to the key without the language,
which nothing read any more; it writes the query's own key.

backend/tests/test_plex_lang.py

@@ -12,7 +12,18 @@ import app.state as state
 from app.auth import current_user
 from app.db import get_db
 from app.main import app
-from app.models import PlexItem, PlexLibrary, User
+from app import sysconfig
+from app.models import (
+    PlexCollection,
+    PlexItem,
+    PlexLibrary,
+    PlexPlaylist,
+    PlexPlaylistItem,
+    PlexSeason,
+    PlexShow,
+    PlexState,
+    User,
+)
 
 
 def _seed(db):
@@ -162,3 +173,99 @@ def test_rating_filter_prefers_imdb_over_audience(db, monkeypatch, override_dep)
     assert "High IMDb" in _titles(hit)
     miss = _get(db, user, monkeypatch, override_dep, "/api/plex/library?scope=movie&rating_min=9")
     assert "High IMDb" not in _titles(miss)
+
+
+def _seed_rails(db):
+    """`_seed` plus what every card rail reads: a HU-overlaid show with a HU-overlaid episode in
+    progress, a cast name, a collection, and a playlist holding the movie and the episode."""
+    user = _seed(db)
+    m1 = db.query(PlexItem).filter_by(rating_key="m1").one()
+    m1.cast_names = ["Anna"]
+    m1.collection_keys = ["c1"]
+    db.add(PlexCollection(rating_key="c1", library_id=m1.library_id, title="Coll"))
+    tv = PlexLibrary(plex_key="2", title="TV Shows", kind="show", enabled=True)
+    db.add(tv)
+    db.flush()
+    show = PlexShow(
+        rating_key="s1",
+        library_id=tv.id,
+        title="English Show",
+        title_hu="Magyar Sorozat",
+        added_at=datetime(2024, 1, 3, tzinfo=timezone.utc),
+    )
+    db.add(show)
+    db.flush()
+    season = PlexSeason(rating_key="s1s1", show_id=show.id, title="Season 1", season_number=1)
+    db.add(season)
+    db.flush()
+    ep = PlexItem(
+        rating_key="e1",
+        library_id=tv.id,
+        kind="episode",
+        show_id=show.id,
+        season_id=season.id,
+        title="Pilot",
+        title_hu="Kezdet",
+        season_number=1,
+        episode_number=1,
+        duration_s=3000,
+    )
+    db.add(ep)
+    db.flush()
+    db.add(
+        PlexState(
+            user_id=user.id,
+            item_id=ep.id,
+            status="in_progress",
+            position_seconds=600,
+            progress_updated_at=datetime(2024, 2, 1, tzinfo=timezone.utc),
+        )
+    )
+    pl = PlexPlaylist(user_id=user.id, title="Mix")
+    db.add(pl)
+    db.flush()
+    db.add_all(
+        [
+            PlexPlaylistItem(playlist_id=pl.id, item_id=m1.id, position=0),
+            PlexPlaylistItem(playlist_id=pl.id, item_id=ep.id, position=1),
+        ]
+    )
+    sysconfig.set_value(db, "plex_enabled", True)
+    db.commit()
+    return user, pl.id
+
+
+def test_every_card_rail_serves_the_readers_language(db, monkeypatch, override_dep):
+    """E30: the grid spoke Hungarian, but every other card surface (Discover rails, show page,
+    search, collection, person page, playlist, Home) served the English title to a HU reader."""
+    user, pid = _seed_rails(db)
+
+    def get(path):
+        return _get(db, user, monkeypatch, override_dep, path).json()
+
+    def titles(cards):
+        return {c["title"] for c in cards}
+
+    rails = get("/api/plex/discover?scope=both&lang=hu")
+    assert {"Magyar Cím", "Magyar Sorozat"} <= titles(rails["recently_added"])
+    assert "Magyar Sorozat" in titles(rails["continue_watching"])
+    assert titles(get("/api/plex/show/s1?lang=hu")["seasons"][0]["episodes"]) == {"Kezdet"}
+    found = get("/api/plex/library?scope=both&lang=hu&q=Kezdet")["episodes"]
+    assert [(e["title"], e["show_title"]) for e in found] == [("Kezdet", "Magyar Sorozat")]
+    assert titles(get("/api/plex/collections/c1?lang=hu")["items"]) == {"Magyar Cím"}
+    assert titles(get("/api/plex/person/Anna?lang=hu")["movies"]) == {"Magyar Cím"}
+    items = get(f"/api/plex/playlists/{pid}?lang=hu")["items"]
+    assert [(c["title"], c.get("show_title")) for c in items] == [
+        ("Magyar Cím", None),
+        ("Kezdet", "Magyar Sorozat"),
+    ]
+    home = get("/api/discover?lang=hu")["continue_watching"]
+    plex = [c for c in home if c["source"] == "plex"]
+    assert [c["title"] for c in plex] == ["Magyar Sorozat"]
+    assert plex[0]["next"]["title"] == "Kezdet"
+    # EN (the default) still serves English on the same rails.
+    items = get(f"/api/plex/playlists/{pid}")["items"]
+    assert [(c["title"], c.get("show_title")) for c in items] == [
+        ("English Title", None),
+        ("Pilot", "English Show"),
+    ]