feat: API improvements

[isTravis]

Infrastructure improvements to handle large collections and reduce server load.

Changes

• Chunked upload protocol — New multi-request upload flow for large version pushes (>100MB). Clients start a session, append records in batches (up to 10K per request), then finalize. Streaming hash computation and Postgres temp tables keep memory bounded during finalize.
• Cursor-based pagination — Records endpoint supports ?after=recordId keyset pagination alongside existing offset pagination. Eliminates slow OFFSET scans on large collections.
• CDN file redirects — File download endpoint now returns a 301 to assets.underlay.org instead of proxying bytes through the app server. Visibility checks still run before redirect.
• Performance indexes — Added indexes on collections(account_id), records(version_id, type), and version_files(file_hash).
• Migration locking — migrate.ts uses a Postgres advisory lock to prevent race conditions when multiple replicas start simultaneously.

New

• Tables: upload_sessions, upload_records
• Route: src/api/routes/uploads.ts
• Migrations: 0002_harsh_madrox.sql, 0003_cool_leper_queen.sql