feat(training): add segmentation training pipeline and complete Part 6

- Add /segtrain endpoint to OCR service (ZIP upload, ketos.segtrain,
  backup rotation, in-process model reload)
- Add segtrainModel() to OcrClient and RestClientOcrClient (10-min timeout,
  X-Training-Token header)
- Add SegmentationTrainingExportService: PAGE XML export with polygon
  de-normalization and per-page PNG rendering via PDFBox
- Add GET /api/ocr/segmentation-training-data/export endpoint
- Make TranscriptionBlock.text nullable for segmentation-only blocks
  (V31 migration)
- Add Paraglide i18n translation keys for all training UI strings (de/en/es)
- Pass source prop from TranscriptionEditView to TranscriptionBlock

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

backend/src/main/resources/db/migration/V31__make_transcription_block_text_nullable.sql

@@ -0,0 +1,5 @@
+-- Intentional: segmentation-only blocks have no text.
+-- This migration is irreversible without a data cleanup step
+-- (cannot re-add NOT NULL if null rows exist).
+ALTER TABLE transcription_blocks ALTER COLUMN text DROP NOT NULL;
+ALTER TABLE transcription_blocks ALTER COLUMN text SET DEFAULT '';