feat(regions): image_region storage + service for the crop pipeline (#114 slice 2)

The storage backbone both crop jobs write to and read from. image_region =
normalized bbox (rx/ry/rw/rh) + kind ('face'/'figure' → CCIP character id;
'concept' → SigLIP head bag) + the crop's embedding (nullable Vector(768) CCIP /
Vector(1152) SigLIP, one per kind) + version stamps for compute-once gating. The
bbox doubles as grounded-tag provenance. Migration 0061.

RegionService.replace_regions (scoped BY KIND so the figure + concept pipelines
don't clobber each other) + get_regions — the GPU agent's results endpoint will
call the writer; the character matcher + bag scorer read. Server-side, no GPU.

Tests: replace/get round-trip, kind-scoped replacement, CCIP vector round-trip.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

backend/app/models/__init__.py

@@ -15,6 +15,7 @@ from .head_training_run import HeadTrainingRun
 from .image_prediction import ImagePrediction
 from .image_provenance import ImageProvenance
 from .image_record import ImageRecord
+from .image_region import ImageRegion
 from .import_batch import ImportBatch
 from .import_settings import ImportSettings
 from .import_task import ImportTask
@@ -60,6 +61,7 @@ __all__ = [
     "ImageRecord",
     "ImagePrediction",
     "ImageProvenance",
+    "ImageRegion",
     "Tag",
     "TagKind",
     "image_tag",