feat(ocr): show translated progress messages during OCR processing

Backend sends progress codes (PREPARING, LOADING, ANALYZING,
CREATING_BLOCKS:N, DONE:N, ERROR) via OcrJob.progressMessage.
Frontend translates them via Paraglide (de/en/es) and displays
below the spinner.

- V27 migration: adds progress_message column to ocr_jobs
- OcrAsyncRunner updates progress at each phase
- Poll interval reduced to 2s for snappier updates

Refs #226

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

backend/src/main/resources/db/migration/V27__add_progress_message_to_ocr_jobs.sql

@@ -0,0 +1 @@
+ALTER TABLE ocr_jobs ADD COLUMN progress_message TEXT;