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/email.py

@@ -83,6 +83,19 @@ def send_access_approved(email: str) -> bool:
     return _send([email], "You're in — your Siftlode access is approved", body, reply_to=admin)
 
 
+def send_access_denied(email: str) -> bool:
+    admin = _admin_contact()
+    body = (
+        "Hi,\n\n"
+        "Thanks for your interest in Siftlode. After review, your access request wasn't approved "
+        "this time.\n\n"
+        "If you think this was a mistake, you're welcome to reply and let us know.\n\n"
+        "— Siftlode"
+        + (f"\n\nQuestions? Just reply to this email ({admin})." if admin else "")
+    )
+    return _send([email], "About your Siftlode access request", body, reply_to=admin)
+
+
 def send_role_changed(to: str, role: str, operator: str | None) -> bool:
     line = (
         "You've been granted admin access on Siftlode — you can now manage settings, the "