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/channels.py

@@ -706,7 +706,7 @@ def attach_tag(
     _user_subscription(db, user, channel_id)
     # Only the user's own tags may be attached as personal links (owned_or_404 → 404 if the tag
     # is missing OR belongs to someone else, without leaking which).
-    owned_or_404(db, Tag, body.tag_id, user, detail="Unknown tag")
+    owned_or_404(db, Tag, body.tag_id, user, what="tag")
     exists = db.execute(
         select(ChannelTag).where(
             ChannelTag.channel_id == channel_id,