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/alembic/versions/0017_notifications.py

@@ -0,0 +1,59 @@
+"""per-user notification inbox (phase 1)
+
+Revision ID: 0017_notifications
+Revises: 0016_video_maintenance
+Create Date: 2026-06-18
+
+Adds the durable, server-backed per-user notification table behind the inbox module. The
+JSON payload column is named `data` (SQLAlchemy's declarative Base reserves `metadata`).
+First producer is the maintenance job's batched "videos removed" notice. Coexists with the
+existing client-side transient bell.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0017_notifications"
+down_revision: Union[str, None] = "0016_video_maintenance"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "notifications",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column(
+            "user_id",
+            sa.Integer(),
+            sa.ForeignKey("users.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column("type", sa.String(length=32), nullable=False),
+        sa.Column("title", sa.String(length=255), nullable=False),
+        sa.Column("body", sa.Text(), nullable=True),
+        sa.Column("data", sa.JSON(), nullable=True),
+        sa.Column("read", sa.Boolean(), nullable=False, server_default="false"),
+        sa.Column("dismissed", sa.Boolean(), nullable=False, server_default="false"),
+        sa.Column(
+            "created_at", sa.DateTime(timezone=True), server_default=sa.func.now()
+        ),
+    )
+    op.create_index("ix_notifications_user_id", "notifications", ["user_id"])
+    op.create_index("ix_notifications_type", "notifications", ["type"])
+    op.create_index("ix_notifications_read", "notifications", ["read"])
+    op.create_index("ix_notifications_created_at", "notifications", ["created_at"])
+    # Fast unread-badge count + inbox listing per user.
+    op.create_index(
+        "ix_notifications_user_read", "notifications", ["user_id", "read"]
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_notifications_user_read", table_name="notifications")
+    op.drop_index("ix_notifications_created_at", table_name="notifications")
+    op.drop_index("ix_notifications_read", table_name="notifications")
+    op.drop_index("ix_notifications_type", table_name="notifications")
+    op.drop_index("ix_notifications_user_id", table_name="notifications")
+    op.drop_table("notifications")