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/tests/test_playlist_move.py

@@ -139,11 +139,49 @@ def test_a_list_that_is_not_yours_answers_with_a_reason_the_ui_can_translate(
 
     r = as_user(user).delete(f"/api/playlists/{theirs.id}/items/v1")
     assert r.status_code == 404
-    assert r.json()["detail"] == {"code": "playlist", "reason": "not_found"}
+    assert r.json()["detail"] == {"code": "not_found", "reason": "playlist"}
     db.expire_all()
     assert _ids(db, theirs.id) == ["v1"]
 
 
+def test_a_move_into_a_list_that_is_gone_names_the_target_not_the_list_on_screen(
+    db, make_user, as_user
+):
+    # Both lists 404'd with the same reason, which the UI reads as "THIS playlist doesn't exist any
+    # more" — said about the source the reader is looking at, which is still there.
+    user = make_user()
+    _seed_videos(db, ["v1"])
+    src = _playlist(db, user, "Mine", ["v1"])
+    gone = _playlist(db, user, "Gone", [])
+    gone_id = gone.id
+    db.delete(gone)
+    db.commit()
+
+    r = _move(as_user(user), src.id, "v1", gone_id)
+    assert r.status_code == 404
+    assert r.json()["detail"] == {"code": "not_found", "reason": "move_target"}
+    db.expire_all()
+    assert _ids(db, src.id) == ["v1"]
+
+
+def test_a_refused_playlist_action_answers_with_a_reason_the_ui_can_translate(
+    db, make_user, as_user
+):
+    # The rest of the router's refusals were bare English too ("The Watch later list can't be
+    # deleted.", "YouTube delete failed.", ...), shown verbatim under the Hungarian UI.
+    user = make_user()
+    client = as_user(user)
+    wl = client.post("/api/playlists/watch-later", json={"video_id": "nope"})
+    assert wl.status_code == 404
+    assert wl.json()["detail"] == {"code": "not_found", "reason": "video"}
+    _seed_videos(db, ["v1"])
+    saved = client.post("/api/playlists/watch-later", json={"video_id": "v1"})
+    wl_id = saved.json()["playlist_id"]
+    r = client.delete(f"/api/playlists/{wl_id}")
+    assert r.status_code == 400
+    assert r.json()["detail"] == {"code": "playlist", "reason": "watch_later_undeletable"}
+
+
 def test_move_marks_both_linked_sides_dirty(db, make_user, as_user):
     """A move edits two YouTube mirrors at once — the source lost an item, the target gained one.
     Marking only the source is the asymmetric fix this repo keeps re-learning."""