fix(notifications): paginate and index the notification list endpoints

[tagpro]

Problem

The notifications tab is very slow to load. Both list endpoints return the entire notification table:

# media_manager/notification/repository.py
stmt = select(Notification).order_by(Notification.timestamp.desc())   # no limit/offset

So every visit to the tab fetches every row, pydantic-validates each one, serialises the whole list to JSON and ships it to the browser. On the instance where I hit this, the notification table had ~437,000 rows — the tab was loading all of them on every page load and on every 30s poll.

The table also has no index beyond the primary key on id, so the ORDER BY timestamp DESC sorts the whole table on every load, and the unread query (WHERE read = false) is a full scan.

Changes

Backend

• GET /notification and GET /notification/unread now take limit (default 50, le=200) and offset (ge=0), threaded through the service into .limit()/.offset() in the SQL. Ordering is unchanged (timestamp descending).
• The total number of matching notifications is returned in an X-Total-Count response header (and exposed via CORS) so the UI knows how many pages there are.
• GET /notification takes an optional read filter. The UI previously built its "read" list by fetching everything and filtering client-side, which is not possible once the endpoint is paginated.
• New migration (b3d51c7fa9e2, chained off e60ae827ed98) adds an index on timestamp DESC for the ORDER BY, and a partial index (WHERE read = false) for the unread query.
• New PATCH /notification/read marks every unread notification as read in a single UPDATE. "Mark All as Read" used to work by looping a PATCH over every notification the client held — that only worked because the client held all of them, and would silently mark just the current page now.

Frontend

• The notifications page loads a page of 50 with a "Load more" button and a "Showing N of M" count, for both the unread and read sections. Read notifications — the bulk of the table — are not fetched at all until that section is expanded.

Tests

• Repository tests covering: limit caps the number returned, offset skips correctly, ordering is timestamp descending, the unread query returns only unread rows and respects limit/offset, and the bulk mark-as-read reaches past the loaded page. Seeded with more rows than the page size so pagination is actually exercised.
• This adds a small pytest harness (tests/, async SQLite via aiosqlite, plus a test dependency group), since there wasn't one yet.

Not in this PR

The reason that instance had 437k notifications is a separate root cause: identical failure notifications are written every import cycle, so the table floods. That's worth fixing on its own; this PR is limited to making the endpoint bounded and indexed, which is what makes the tab slow regardless of how the rows got there.

Verification

• uv run pytest — 14 passed
• uv run ruff check ./media_manager — clean
• npm run lint (in web/) — clean
• Drove the router end-to-end against a seeded DB: default page returns 50 rows with X-Total-Count, limit=500 is rejected with 422, offset pages don't overlap, the unread page never contains read rows, and PATCH /notification/read empties the unread list.
• alembic upgrade e60ae827ed98:b3d51c7fa9e2 --sql renders the expected DDL, including the partial index.

🤖 Generated with Claude Code

https://claude.ai/code/session_019wrZPxJDip5x3mzjWnpwos

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