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>

api/tests/integration/test_serving.py

@@ -1,9 +1,11 @@
 """
-T055 — GET /api/v1/images/{id}/file → 302 with Location header
+T055 — GET /api/v1/images/{id}/file → 200 with binary content, ETag, Cache-Control
 T056 — /file for unknown ID → 404 image_not_found
+T057 — /file response exposes no storage-specific details
 """
 import io
 import uuid
+
 import pytest
 
 
@@ -17,20 +19,23 @@ def _minimal_webp() -> bytes:
 
 
 @pytest.mark.asyncio
-async def test_file_redirect_returns_302(client):
+async def test_file_returns_200_with_content(client):
     data = _minimal_webp()
     upload = await client.post(
         "/api/v1/images",
         files={"file": ("img.webp", io.BytesIO(data), "image/webp")},
     )
     assert upload.status_code in (200, 201)
-    image_id = upload.json()["id"]
+    upload_body = upload.json()
+    image_id = upload_body["id"]
+    image_hash = upload_body["hash"]
 
-    # Don't follow redirects
-    response = await client.get(f"/api/v1/images/{image_id}/file", follow_redirects=False)
-    assert response.status_code == 302
-    assert "Location" in response.headers
-    assert response.headers["Location"]  # must not be empty
+    response = await client.get(f"/api/v1/images/{image_id}/file")
+    assert response.status_code == 200
+    assert response.headers["content-type"].startswith("image/")
+    assert response.headers["etag"] == f'"{image_hash}"'
+    assert "immutable" in response.headers["cache-control"]
+    assert len(response.content) > 0
 
 
 @pytest.mark.asyncio
@@ -39,3 +44,21 @@ async def test_file_unknown_id_returns_404(client):
     assert response.status_code == 404
     body = response.json()
     assert body["code"] == "image_not_found"
+
+
+@pytest.mark.asyncio
+async def test_file_response_exposes_no_storage_details(client):
+    data = _minimal_webp()
+    upload = await client.post(
+        "/api/v1/images",
+        files={"file": ("img.webp", io.BytesIO(data), "image/webp")},
+    )
+    assert upload.status_code in (200, 201)
+    image_id = upload.json()["id"]
+
+    response = await client.get(f"/api/v1/images/{image_id}/file")
+    assert response.status_code == 200
+    assert "location" not in response.headers
+    assert "minio" not in response.text.lower()
+    assert "s3://" not in response.text.lower()
+    assert "amazonaws.com" not in response.text.lower()