feat(ocr): add training history + POST /train + GET /training-info endpoints

- OcrTrainingRun entity + V30 migration (partial unique index prevents
  concurrent runs at DB level)
- OcrTrainingService: concurrent-run guard, 5-block threshold, MDC log
  correlation, orphan recovery on ApplicationReadyEvent
- POST /api/ocr/train (ADMIN) + GET /api/ocr/training-info (ADMIN)
- TRAINING_ALREADY_RUNNING ErrorCode
- 6 OcrTrainingServiceTest + 6 OcrControllerTest tests for the new endpoints

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

backend/src/main/resources/db/migration/V30__add_ocr_training_runs.sql

@@ -0,0 +1,16 @@
+CREATE TABLE ocr_training_runs (
+    id             UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    status         VARCHAR(20)  NOT NULL DEFAULT 'RUNNING',
+    block_count    INT          NOT NULL,
+    document_count INT          NOT NULL,
+    model_name     VARCHAR(100) NOT NULL,
+    error_message  TEXT,
+    triggered_by   UUID REFERENCES users(id) ON DELETE SET NULL,
+    created_at     TIMESTAMPTZ  NOT NULL DEFAULT now(),
+    completed_at   TIMESTAMPTZ
+);
+
+-- Enforce single active run at the DB layer (application check is the UX layer)
+CREATE UNIQUE INDEX idx_ocr_training_runs_one_running
+    ON ocr_training_runs (status)
+    WHERE status = 'RUNNING';