Add unified IG and Twitter action log endpoint

[mircats98gpt]

Closes #155.

Summary

• Adds GET /api/actions/unified for a merged Instagram + Twitter action log view.
• Returns a consistent action shape with platform, action, timestamp, status, and metadata.
• Adds multi-platform filtering to the action log service so pagination reflects the unified IG/Twitter set.
• Documents the endpoint in the API docs list.

Validation

• pnpm --filter @riona/api typecheck
• pnpm --filter @riona/api exec jest src/services/actionLog.test.ts --runInBand
• pnpm --filter @riona/api exec jest src/routes/api.test.ts --runInBand --forceExit

Note: the route test suite keeps an existing rate-limiter timer open, so I used --forceExit after assertions passed.