feat(transcription): add source/reviewed fields for training pipeline

- BlockSource enum: MANUAL, OCR
- V26 migration adds source + reviewed columns to transcription_blocks
- OcrService sets source=OCR when creating blocks
- TranscriptionService.reviewBlock() toggles the reviewed flag
- PUT /api/documents/{id}/transcription-blocks/{blockId}/review endpoint
- 5 new tests: reviewBlock toggle/untoggle/notfound, controller,
  OcrService source=OCR verification

The reviewed flag enables the Kraken fine-tuning pipeline: only blocks
marked as reviewed by a human are exported as training data.

Refs #226

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V26__add_source_and_reviewed_to_transcription_blocks.sql

@@ -0,0 +1,2 @@
+ALTER TABLE transcription_blocks ADD COLUMN source VARCHAR(10) NOT NULL DEFAULT 'MANUAL';
+ALTER TABLE transcription_blocks ADD COLUMN reviewed BOOLEAN NOT NULL DEFAULT FALSE;