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.

frontend/src/i18n/locales/en/errors.json

@@ -4,6 +4,17 @@
   "validation": "Some of the values you entered weren't valid. Please check them and try again.",
   "server": "Server error",
   "syncCooldown": "You've synced a few times just now — give it a few minutes. The YouTube budget is shared with everyone on this instance, and background syncing keeps running either way.",
+  "notFound": {
+    "generic": "This item doesn't exist any more.",
+    "view": "This view doesn't exist any more.",
+    "tag": "This tag doesn't exist any more.",
+    "pin": "This isn't on your comfort shelf any more.",
+    "playlist": "This playlist doesn't exist any more.",
+    "move_target": "The playlist you're moving it to doesn't exist any more.",
+    "video": "This video isn't in the library any more.",
+    "invite": "This invite doesn't exist any more.",
+    "user": "This user doesn't exist any more."
+  },
   "ok": "OK",
   "offline": {
     "title": "Connection lost",

frontend/src/i18n/locales/en/playlists.json

@@ -78,7 +78,14 @@
   "moveFailed": "Couldn't move the video.",
   "removeFailed": "Couldn't remove the video.",
   "errors": {
-    "not_found": "This playlist doesn't exist any more."
+    "name_required": "Give the playlist a name.",
+    "youtube_read_scope": "Connect YouTube (read access) to sync your playlists.",
+    "not_on_youtube": "This playlist isn't on YouTube.",
+    "youtube_failed": "YouTube didn't accept that just now. Please try again.",
+    "watch_later_not_pushable": "The Watch later list can't be sent to YouTube.",
+    "watch_later_undeletable": "The Watch later list can't be deleted.",
+    "youtube_editing_off": "Enable YouTube editing in Settings first.",
+    "quota_exceeded": "Not enough YouTube quota left today for this sync."
   },
   "moveErrors": {
     "not_in_playlist": "That video isn't in this playlist any more — it changed somewhere else.",

frontend/src/i18n/locales/hu/errors.json

@@ -4,6 +4,17 @@
   "validation": "Néhány megadott érték érvénytelen volt. Ellenőrizd, majd próbáld újra.",
   "server": "Szerverhiba",
   "syncCooldown": "Az imént többször is szinkronizáltál — várj pár percet. A YouTube-keret közös az instance összes felhasználójával, a háttérszinkron pedig így is fut tovább.",
+  "notFound": {
+    "generic": "Ez az elem már nem létezik.",
+    "view": "Ez a nézet már nem létezik.",
+    "tag": "Ez a címke már nem létezik.",
+    "pin": "Ez már nincs a komfort polcodon.",
+    "playlist": "Ez a lejátszási lista már nem létezik.",
+    "move_target": "A lejátszási lista, ahová áthelyeznéd, már nem létezik.",
+    "video": "Ez a videó már nincs a könyvtárban.",
+    "invite": "Ez a meghívó már nem létezik.",
+    "user": "Ez a felhasználó már nem létezik."
+  },
   "ok": "OK",
   "offline": {
     "title": "Megszakadt a kapcsolat",

frontend/src/i18n/locales/hu/playlists.json

@@ -78,7 +78,14 @@
   "moveFailed": "Nem sikerült áthelyezni a videót.",
   "removeFailed": "Nem sikerült eltávolítani a videót.",
   "errors": {
-    "not_found": "Ez a lejátszási lista már nem létezik."
+    "name_required": "Adj nevet a lejátszási listának.",
+    "youtube_read_scope": "A listák szinkronizálásához kapcsold össze a YouTube-ot (olvasási hozzáférés).",
+    "not_on_youtube": "Ez a lejátszási lista nincs fent a YouTube-on.",
+    "youtube_failed": "A YouTube most nem fogadta el. Próbáld újra.",
+    "watch_later_not_pushable": "A „Megnézem később” listát nem lehet a YouTube-ra küldeni.",
+    "watch_later_undeletable": "A „Megnézem később” lista nem törölhető.",
+    "youtube_editing_off": "Előbb engedélyezd a YouTube-szerkesztést a Beállításokban.",
+    "quota_exceeded": "Mára nem maradt elég YouTube-keret ehhez a szinkronhoz."
   },
   "moveErrors": {
     "not_in_playlist": "Ez a videó már nincs ebben a listában — időközben máshol megváltozott.",