feat(ux): R11 feedback symmetry — Plex/download/rollback/deny notices

Give equal-weight actions equal-weight feedback (R11).

S1 Plex error toasts: the watched-toggles (card / season mark-all / episode /
info-panel / show mark-all) used to fail silently on 403/404/429; each now surfaces
the server reason via a shared notifyActionError() + resyncs the optimistic flip.
The playback auto-mark-watched stays intentionally silent (a mid-play toast is
noise; the next sync corrects it). New generic lib/actionErrors.ts (Plex + saved
views reuse it; notifyYouTubeActionError delegates its non-403 case to it).

S2 download-done inbox notification: worker.py emits a best-effort type=download_done
notice to the job owner on real completion (regular + edit jobs, not instant
cache-hits), rendered/localized in the inbox with an Open-Downloads link.

S3 optimistic rollbacks: SavedViewsWidget create/rename/toggle-default/remove/reorder
and the inbox revert-state no longer swallow failures — they toast + refetch to
resync (the reorder's optimistic order reverts to server truth).

S4 access-request deny: deny_invite now emails the requester (send_access_denied),
mirroring approve — no more silent limbo.

backend/app/routes/admin.py

@@ -105,10 +105,15 @@ def approve_invite(
 @router.post("/invites/{invite_id}/deny")
 def deny_invite(
     invite_id: int,
+    background: BackgroundTasks,
     admin: User = Depends(admin_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    return _serialize(_decide(db, invite_id, admin, approved=False))
+    # Symmetry with approve: the requester hears back either way, instead of a silent limbo. No
+    # account is activated, so this is an email only (the requester has no in-app inbox yet).
+    inv = _decide(db, invite_id, admin, approved=False)
+    background.add_task(email_mod.send_access_denied, inv.email)
+    return _serialize(inv)
 
 
 class AddInviteIn(BaseModel):