TranscriptionBlock entity with @Version optimistic locking
TranscriptionBlockVersion for edit history
TranscriptionService facade: CRUD, reorder, version history
TranscriptionBlockController: REST endpoints under /api/documents/{docId}/transcription-blocks
DTOs: Create, Update, Reorder
ErrorCode: TRANSCRIPTION_BLOCK_NOT_FOUND, TRANSCRIPTION_BLOCK_CONFLICT
DocumentComment: add block_id field for block-level comment threads
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>