feat(schema): align API with lexicon v0.3.0 (#164)
- Content field now uses union object { $type, value } in PDS records
- Rename topic createdAt → publishedAt (DB column + all references)
- Remove contentFormat from topics and replies (DB + validation + routes)
- Add optional site field to topics
- Backward-compatible content extraction in firehose indexers
- Migration 0011 handles column rename, drop, and addition
- Update all test fixtures for new record format
Closes singi-labs/barazo-workspace#54
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