fix(ocr): add partial unique index and align SenderModelServiceTest with suite style

Add V42 partial unique index on ocr_training_runs(person_id) WHERE status='QUEUED'
to enforce the per-person queued coalescing guarantee at the DB level. Also adds
@ExtendWith(MockitoExtension.class) to SenderModelServiceTest for consistency with
the rest of the service test suite, with lenient() on the shared txTemplate stub.

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

backend/src/main/resources/db/migration/V42__add_queued_per_person_unique_index.sql

@@ -0,0 +1,3 @@
+CREATE UNIQUE INDEX idx_training_runs_queued_per_person
+    ON ocr_training_runs(person_id)
+    WHERE status = 'QUEUED';