test(notifications): add unit tests for dmNotifications task

[dylanjeffers]

Summary

Adds the first test coverage for apps/notifications/src/tasks/dmNotifications.ts (previously zero tests): 23 unit tests in src/tasks/__tests__/dmNotifications.test.ts.

Unlike the existing integration suite (which needs template Postgres DBs + Redis via audius-compose), these are pure unit tests: a small chainable Knex mock records each query's builder calls / raw SQL / bindings, and Redis is an in-memory map. They run standalone with npx jest src/tasks/__tests__/dmNotifications.test.ts and also under the normal npm test in CI.

Coverage

getUnreadMessages / getUnreadReactions

• returns the unread rows; empty results
• query shape: excludes blasts (blast_id null), aliases created_at/updated_at as timestamp, min/max cursor bounds, last_active_at guard, excludes own messages/reactions

getNewBlasts

• messages carry a populated timestamp (regression: created_at AS timestamp alias from fix(notifications): send blast push notifications again #73) and a computed chat_id
• chat_allowed applies to every batch — asserts the parenthesized AND (?::INTEGER IS NULL OR to_user_id > ?) guard from fix(notifications): send blast push notifications again #73
• cursor behavior: advances user-id cursor to last recipient, moves to next blast + resets user cursor when a batch is exhausted, holds position at end of table, first-run fast-forward without backfill flood, empty table

sendDMNotifications

• pushes sent for messages, reactions, and blasts; all Redis cursors advanced
• notifications processed in ascending timestamp order
• age guard (dmNotificationMaxAgeMs): too-old messages/reactions skipped while cursors still advance; fresh ones in the same run still sent; 0 disables the guard
• blast exemption from the age guard (blastSet): a blast older than the cutoff still reaches its whole audience while an equally old DM is skipped
• empty run initializes cursors; cached Redis timestamps become the query min-bounds; a processNotification failure is logged without advancing cursors (retried next tick)

Also exports getUnreadMessages, getUnreadReactions, getNewBlasts from dmNotifications.ts so they can be tested directly (no behavior change).

Note: written with jest, not vitest — this app's suite runs on jest (npm test), and adding a second runner would leave these tests out of CI.

Test plan

• cd apps/notifications && npx jest src/tasks/__tests__/dmNotifications.test.ts — 23/23 pass
• npm run typecheck — clean
• npx eslint on both changed files — no errors

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