Add unified Instagram and Twitter actions API

[eldenizfamilyanskicode]

Closes #155

Summary

• Added GET /api/actions/unified as an authenticated endpoint.
• Merges Instagram and Twitter action logs into one timestamp-sorted feed.
• Normalizes each entry to platform, action, timestamp, status, and
  metadata.
• Supports ?limit= and ?offset= pagination params and returns pagination
  metadata.

Testing

pnpm --filter @riona/api exec jest src/routes/api.test.ts --runInBand --forceExit
pnpm --filter @riona/api typecheck
pnpm --filter @riona/api exec eslint src/routes/api.ts src/routes/api.test.ts
pnpm exec prettier --check apps/api/src/routes/api.ts apps/api/src/routes/api.test.ts

Notes

• The endpoint reuses the existing listActionLogs service rather than adding
  another log store.
• GitHub actions are intentionally out of scope because the issue asked to
  focus on Instagram and Twitter.