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>

backend/src/main/resources/application.yaml

@@ -77,3 +77,8 @@ app:
       tags: 10
       summary: 11
       transcription: 13
+
+ocr:
+  sender-model:
+    activation-threshold: 100
+    retrain-delta: 50

backend/src/main/resources/db/migration/V40__add_sender_models.sql

@@ -0,0 +1,10 @@
+CREATE TABLE sender_models (
+    id                          UUID        PRIMARY KEY DEFAULT gen_random_uuid(),
+    person_id                   UUID        NOT NULL UNIQUE REFERENCES persons(id) ON DELETE CASCADE,
+    model_path                  TEXT        NOT NULL,
+    accuracy                    DOUBLE PRECISION,
+    cer                         DOUBLE PRECISION,
+    corrected_lines_at_training INT         NOT NULL DEFAULT 0,
+    created_at                  TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at                  TIMESTAMPTZ NOT NULL DEFAULT now()
+);

backend/src/main/resources/db/migration/V41__add_person_to_training_runs.sql

@@ -0,0 +1,2 @@
+ALTER TABLE ocr_training_runs
+    ADD COLUMN person_id UUID REFERENCES persons(id) ON DELETE SET NULL;