feat(ocr): wire SenderModelService into OcrAsyncRunner; stage missing foundational files

OcrAsyncRunner now passes the per-sender model path to streamBlocks for
HANDWRITING_KURRENT documents. processDocument replaced extractBlocks
with streamBlocks + AtomicReference, removing the unchecked raw-array
pattern.

Also stages all previously uncommitted foundational files for this
feature: SenderModel entity, SenderModelRepository, Flyway migrations
V40/V41, updated OcrClient/RestClientOcrClient streaming API,
TrainingDataExportService.exportForSender, TranscriptionService Kurrent
hook, application.yaml OCR config, and frontend i18n/test additions.

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

frontend/messages/de.json

@@ -561,6 +561,11 @@
 	"transcription_block_segmentation_only": "Nur Segmentierung",
 	"training_chip_kurrent": "Kurrent-Erkennung",
 	"training_chip_segmentation": "Segmentierung",
+	"training_col_type": "Typ",
+	"training_type_base": "Basis",
+	"training_type_personalized": "Personalisiert",
+	"training_col_person": "Absender",
+	"training_status_queued": "Warteschlange",
 	"mission_control_heading": "Was braucht Aufmerksamkeit?",
 	"mission_control_segmentation_heading": "Text markieren",
 	"mission_control_segmentation_description": "Textbereiche markieren — keine Vorkenntnisse nötig",