feat(notifications): durable per-user inbox (P1) + maintenance schema

Add a server-backed notification center that coexists with the client-side
transient bell: a per-user `notifications` table (type/title/body/data JSON/
read/dismissed), a `/api/me/notifications` CRUD API (list, unread_count, read,
read_all, dismiss, clear), and a left-nav inbox module with a live unread badge
polled via useLiveQuery. Known types render trilingual text from type+data
(English stored text is the fallback); read rows are trimmed past a soft cap.

Also adds the schema the maintenance job builds on: videos.list?part=status
columns (embeddable/privacy_status/upload_status) and the validation lifecycle
columns (last_checked_at, unavailable_since, unavailable_reason).

Page-id validation is centralized in one PAGES source of truth (isPage) so the
new page survives reload without a second allowlist to keep in sync.

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

@@ -0,0 +1,16 @@
+{
+  "navLabel": "Notifications",
+  "title": "Notifications",
+  "subtitle": "Updates from your library and the system.",
+  "empty": "You're all caught up.",
+  "markAllRead": "Mark all read",
+  "clearAll": "Clear all",
+  "markRead": "Mark read",
+  "dismiss": "Dismiss",
+  "andMore": "and {{count}} more",
+  "maintenance": {
+    "title": "Videos removed",
+    "body_one": "{{count}} saved or playlisted video was removed because it's no longer available on YouTube.",
+    "body_other": "{{count}} saved or playlisted videos were removed because they're no longer available on YouTube."
+  }
+}