feat(ocr): add OCR infrastructure (interfaces, entities, migrations, DTOs)

- OcrClient + OcrHealthClient interfaces for testable OCR integration
- OcrBlockResult record for OCR engine response mapping
- OcrJob + OcrJobDocument entities with status enums
- V25 migration creates ocr_jobs and ocr_job_documents tables
- Repositories for job and job-document queries
- TriggerOcrDTO, BatchOcrDTO (@Size max=500), OcrStatusDTO
- ErrorCodes: OCR_SERVICE_UNAVAILABLE, OCR_JOB_NOT_FOUND,
  OCR_DOCUMENT_NOT_UPLOADED, OCR_PROCESSING_FAILED

Refs #226

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

backend/src/main/resources/db/migration/V25__add_ocr_job_tables.sql

@@ -0,0 +1,26 @@
+CREATE TABLE ocr_jobs (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    status VARCHAR(20) NOT NULL DEFAULT 'PENDING',
+    total_documents INT NOT NULL,
+    processed_documents INT NOT NULL DEFAULT 0,
+    error_count INT NOT NULL DEFAULT 0,
+    skipped_count INT NOT NULL DEFAULT 0,
+    created_by UUID,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE TABLE ocr_job_documents (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    job_id UUID NOT NULL REFERENCES ocr_jobs(id) ON DELETE CASCADE,
+    document_id UUID NOT NULL REFERENCES documents(id) ON DELETE CASCADE,
+    status VARCHAR(20) NOT NULL DEFAULT 'PENDING',
+    error_message TEXT,
+    current_page INT DEFAULT 0,
+    total_pages INT DEFAULT 0,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_ocr_job_documents_job_id ON ocr_job_documents(job_id);
+CREATE INDEX idx_ocr_job_documents_document_id ON ocr_job_documents(document_id);