feat(training): add document-level training enrollment

- V29 migration: document_training_labels join table
- TrainingLabel enum: KURRENT_RECOGNITION, KURRENT_SEGMENTATION
- Document.trainingLabels @ElementCollection
- DocumentService.addTrainingLabel / removeTrainingLabel
- PATCH /api/documents/{id}/training-labels (WRITE_ALL)
- Auto-enroll on Kurrent OCR trigger (OcrService.startOcr)
- TranscriptionEditView: enrollment chips in panel footer
- JPQL queries updated to use MEMBER OF trainingLabels

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

backend/src/main/resources/db/migration/V29__add_document_training_labels.sql

@@ -0,0 +1,5 @@
+CREATE TABLE document_training_labels (
+    document_id UUID NOT NULL REFERENCES documents(id) ON DELETE CASCADE,
+    label       VARCHAR(50) NOT NULL,
+    PRIMARY KEY (document_id, label)
+);