Scale notification delivery pipeline

[productdevbook]

Summary

• move notification fan-out, tracking, and webhook work onto explicit queue-backed workers
• add deterministic notification job ids, status lifecycle updates, and stable target paging for safer retries at scale
• cache API key validation, add notification targeting/status migrations, and update analytics aggregation paths

Verification

• pnpm build
• pnpm lint
• pnpm typecheck
• pnpm exec vitest run tests/batching.test.ts tests/api-key-cache.test.ts tests/notification-status.test.ts tests/notification-queue.test.ts

Notes

• full pnpm test still has pre-existing failures in provider/crypto-related suites and is not included here