fix(api): a gone row and a refused playlist action are said in the reader's language, and a failure is reported once

Deep-review round 3 of bug hunt 2026-09.

- get_or_404 / owned_or_404 answer a structured {code: "not_found", reason: <what>}
  (errors.notFound.* in HU+EN, generic fallback). The structured-404 fix of round 2 had gone into the
  ad-hoc `_own_playlist` (CLAUDE.md's "Not" pattern) while the shared helpers stayed string-only, so
  views, tags, pins, Plex playlists, invites and users still 404'd in English. `_own_playlist` now
  goes through owned_or_404; admin's hand-rolled user lookups use get_or_404.
- A move into a list that is gone names the TARGET (reason "move_target") — "this playlist doesn't
  exist any more" was read as the list on screen, which still exists.
- Every refusal in the playlists router is a {code: "playlist", reason} (playlists.errors.* HU+EN)
  instead of a bare English string shown verbatim in the Hungarian UI.
- `req` / `uploadReq` mark the errors they already put on screen (the error dialog, the connection
  notice, the 401 sign-out), and notifyActionError skips those. The watched toggles, the saved views
  and a playlist delete toasted a fault the dialog was already showing; Playlists' two hand-written
  status filters now go through the same check.

backend/app/routes/_common.py

@@ -17,12 +17,20 @@ from app.models import User
 M = TypeVar("M", bound=Base)
 
 
-def get_or_404(db: Session, model: type[M], id_: Any, *, detail: str = "Not found") -> M:
+def not_found(what: str = "generic") -> HTTPException:
+    """The 404 for a row that is gone — structured, so the bilingual UI says it in the reader's
+    language (`errors.notFound.<what>`, generic fallback). This is the 404 a user actually meets: the
+    thing was deleted in another tab or on another device. It used to be a bare English string
+    ("Unknown view", "Not found"), which the Hungarian UI toasted verbatim."""
+    return HTTPException(status_code=404, detail={"code": "not_found", "reason": what})
+
+
+def get_or_404(db: Session, model: type[M], id_: Any, *, what: str = "generic") -> M:
     """Fetch a row by primary key or raise 404. Replaces the `db.get(Model, id); if None: 404`
     idiom repeated across routes."""
     obj = db.get(model, id_)
     if obj is None:
-        raise HTTPException(status_code=404, detail=detail)
+        raise not_found(what)
     return obj
 
 
@@ -33,13 +41,13 @@ def owned_or_404(
     user: User,
     *,
     owner_attr: str = "user_id",
-    detail: str = "Not found",
+    what: str = "generic",
 ) -> M:
     """Fetch a row by primary key that `user` owns, or raise 404. A row owned by someone
     else 404s (NOT 403) on purpose — a 403 would confirm the id exists to a stranger.
     Generalizes the near-identical per-route ownership helpers (e.g. plex's
     `_own_playlist_or_404`, the saved-views owner check)."""
-    obj = get_or_404(db, model, id_, detail=detail)
+    obj = get_or_404(db, model, id_, what=what)
     if getattr(obj, owner_attr) != user.id:
-        raise HTTPException(status_code=404, detail=detail)
+        raise not_found(what)
     return obj