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/worker.py

@@ -34,10 +34,31 @@ from app.downloads import edit as editmod
 from app.downloads import errors as dlerrors
 from app.downloads import formats, quota, service, storage
 from app.models import DownloadJob, MediaAsset
+from app.notifications import create_notification
 from app.titles import normalize_title
 
 log = logging.getLogger("siftlode.worker")
 
+
+def _notify_download_done(db, job: DownloadJob | None, title: str | None) -> None:
+    """Post a best-effort 'download ready' notice to the owner's inbox — downloads used to slide to
+    Done silently while scheduler jobs and playlist-push already notified (R11 feedback symmetry).
+    A notification failure must never mask the completed download, so it is isolated + rolled back
+    (mirrors scheduler._notify_done). English title is a fallback; the inbox localizes from `data`."""
+    if not job:
+        return
+    try:
+        create_notification(
+            db,
+            user_id=job.user_id,
+            type="download_done",
+            title=f"Download ready: {title}" if title else "Download ready",
+            data={"title": title},
+        )
+    except Exception:
+        db.rollback()
+        log.exception("failed to write download-done notification for job %s", job.id)
+
 _stop = threading.Event()
 
 
@@ -637,6 +658,7 @@ def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spe
                     # display_name is VARCHAR(255) — clip a long title (see _fill_asset_meta_early).
                     job.display_name = meta.title[:255] if meta.title else None
             db.commit()
+            _notify_download_done(db, job, meta.title)
         log.info("job %s done: %s", job_id, rel)
 
     except (_Aborted, editmod.EditAborted):
@@ -769,6 +791,7 @@ def _process_edit(job_id: int, asset_id: int) -> None:
                 job.progress = 100
                 job.phase = None
             db.commit()
+            _notify_download_done(db, job, asset.title if asset else None)
         log.info("edit job %s done: %s", job_id, rel)
 
     except editmod.EditAborted: