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.

backend/app/models.py

@@ -203,6 +203,22 @@ class Video(Base):
         String(16), default="none", server_default="none", index=True
     )
     enriched_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    # videos.list?part=status fields (populated during enrichment); used by the
+    # maintenance/validation job to decide whether a video is playable anywhere.
+    embeddable: Mapped[bool | None] = mapped_column(Boolean)
+    privacy_status: Mapped[str | None] = mapped_column(String(16))  # public|unlisted|private
+    upload_status: Mapped[str | None] = mapped_column(String(16))  # processed|failed|rejected|deleted
+    # Maintenance/validation lifecycle. last_checked_at drives the rolling re-validation
+    # (least-recently-checked first); unavailable_since marks a video as currently
+    # unplayable (hidden from the feed immediately, hard-deleted after a grace period);
+    # unavailable_reason records why (deleted|private|paywalled|abandoned|stuck_live).
+    last_checked_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), index=True
+    )
+    unavailable_since: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), index=True
+    )
+    unavailable_reason: Mapped[str | None] = mapped_column(String(24))
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now()
     )
@@ -399,3 +415,36 @@ class PlaylistItem(Base):
     )
 
     playlist: Mapped["Playlist"] = relationship(back_populates="items")
+
+
+class Notification(Base):
+    """A durable, server-backed per-user notification (the inbox center, phase 1).
+
+    Distinct from the client-side transient "bell" (toasts kept only in localStorage):
+    these survive reloads and devices, and are produced server-side (first producer = the
+    maintenance/validation job's batched "N saved/playlisted videos removed" notice). The
+    column is named `data` (not `metadata`, which SQLAlchemy's declarative Base reserves):
+    a free-form JSON payload (e.g. the list of removed videos) the UI can render. `read`
+    and `dismissed` are separate: read clears the unread badge but keeps the row in the
+    inbox; dismissed removes it from the default view. Unread rows never auto-expire; read
+    ones are trimmed past a soft per-user cap (see app.notifications.trim_read)."""
+
+    __tablename__ = "notifications"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    user_id: Mapped[int] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE"), index=True
+    )
+    type: Mapped[str] = mapped_column(String(32), index=True)  # e.g. "maintenance"
+    title: Mapped[str] = mapped_column(String(255))
+    body: Mapped[str | None] = mapped_column(Text)
+    data: Mapped[dict | None] = mapped_column(JSON)
+    read: Mapped[bool] = mapped_column(
+        Boolean, default=False, server_default="false", index=True
+    )
+    dismissed: Mapped[bool] = mapped_column(
+        Boolean, default=False, server_default="false"
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), index=True
+    )