Feat: Proxy image content through the API instead of redirecting to MinIO

Replace the presigned-URL redirect (302) in GET /api/v1/images/{id}/file
with a direct proxy that fetches bytes from S3 server-side and returns them
to the client. The browser never contacts the storage backend, eliminating
the /etc/hosts workaround needed in local development.

- StorageBackend: swap get_presigned_url for get(key) -> bytes
- S3StorageBackend: implement get() via aiobotocore get_object
- serve_image_file: return Response with ETag + Cache-Control: immutable
- test_serving: assert 200 + content-type + ETag; add no-storage-details test
- Spec Kit artifacts for feature 002-api-image-proxy

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

.specify/feature.json

@@ -1,3 +1,3 @@
 {
-  "feature_directory": "specs/001-reaction-image-board"
+  "feature_directory": "specs/002-api-image-proxy"
 }