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/app/routes/discover.py

@@ -23,6 +23,7 @@ from app.discover import hero as hero_mod
 from app.discover import pins, rails
 from app.discover.budget import Budget, DEFAULT_BUDGET
 from app.models import ComfortPin, User
+from app.plex.cards import norm_lang
 from app.routes._common import owned_or_404, not_found
 
 router = APIRouter(prefix="/api/discover", tags=["discover"])
@@ -45,6 +46,7 @@ class ReorderIn(BaseModel):
 def get_discover(
     budget: Budget = DEFAULT_BUDGET,
     seed: int = 0,
+    lang: str = "en",
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
@@ -52,13 +54,14 @@ def get_discover(
 
     `resume` and `shelf` are built once and handed to the hero, so the two rails the hero draws from
     are not queried twice for a single page load."""
-    resume = rails.continue_watching(db, user)
+    lang = norm_lang(lang)
+    resume = rails.continue_watching(db, user, lang=lang)
     # Plex off → the shelf rail (and so the hero's comfort tier) goes YouTube-only like the rails
     # beside it; the pins are kept and `GET /pins` still lists them (`pins.shelf`).
-    shelf = pins.shelf(db, user, include_plex=rails.plex_available(db))
+    shelf = pins.shelf(db, user, include_plex=rails.plex_available(db), lang=lang)
     return {
         "budget": budget,
-        "hero": hero_mod.pick(db, user, budget, seed, resume=resume, shelf=shelf),
+        "hero": hero_mod.pick(db, user, budget, seed, resume=resume, shelf=shelf, lang=lang),
         "continue_watching": resume,
         "comfort": shelf,
     }
@@ -80,6 +83,7 @@ def get_pick(
     seed: int = 0,
     exclude: str = "",
     start: int = 0,
+    lang: str = "en",
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
@@ -99,18 +103,25 @@ def get_pick(
     return {
         "budget": budget,
         "hero": hero_mod.pick(
-            db, user, budget, seed, exclude=set(list(skip)[:MAX_EXCLUDE]), start=start
+            db,
+            user,
+            budget,
+            seed,
+            exclude=set(list(skip)[:MAX_EXCLUDE]),
+            start=start,
+            lang=norm_lang(lang),
         ),
     }
 
 
 @router.get("/pins")
 def list_pins(
+    lang: str = "en",
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
     """The shelf on its own — for the shelf editor, which needs it without the rest of the page."""
-    return {"items": pins.shelf(db, user)}
+    return {"items": pins.shelf(db, user, lang=norm_lang(lang))}
 
 
 @router.post("/pins")