feat(topics): AT Protocol-style lookup endpoints (#139)
Add author-scoped endpoints for topics and replies to support
AT Protocol-style URLs (/{handle}/{rkey}) and fix the latent
rkey collision bug in the existing by-rkey endpoint.
- Add GET /api/topics/by-author-rkey/:handle/:rkey
- Add GET /api/replies/by-author-rkey/:handle/:rkey
- Add shared resolveHandleToDid utility (local DB + Bluesky fallback)
- Add authorHandle to POST /api/topics 201 response
- Enrich GET /api/notifications with actorHandle, subjectTitle,
subjectAuthorDid, subjectAuthorHandle, and message fields
- Update cross-post URL builder to AT Protocol-style format
- Add composite indexes on (author_did, rkey) for topics and replies
- Keep existing by-rkey endpoint for backwards compatibility
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