Single-document OCR now creates an OcrJobDocument row so
GET /api/documents/{id}/ocr-status can find running jobs.
OcrAsyncRunner updates the job document status (RUNNING → DONE/FAILED).
Frontend checks OCR status when entering transcription mode —
if a job is running, resumes polling and shows the spinner.
Refs #226
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>