[Spec Kit] Implementation progress

Implements all 88 tasks for the Reaction Image Board (specs/001-reaction-image-board):

- docker-compose.yml: postgres, minio, minio-init, api, ui services with healthchecks
- api/: FastAPI app with SQLAlchemy 2.x async, Alembic migrations, S3/MinIO storage,
  full integration + unit test suite (pytest + pytest-asyncio)
- ui/: Angular 19 standalone app (Library, Upload, Detail, NotFound components)
- .env.example: all required environment variables
- .gitignore: Python, Node, Docker, IDE, .env patterns

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

api/app/storage/backend.py

@@ -0,0 +1,15 @@
+from abc import ABC, abstractmethod
+
+
+class StorageBackend(ABC):
+    @abstractmethod
+    async def put(self, key: str, data: bytes, content_type: str) -> None:
+        """Store object at key with given content type."""
+
+    @abstractmethod
+    async def get_presigned_url(self, key: str, expires_in_seconds: int = 3600) -> str:
+        """Return a pre-signed URL valid for expires_in_seconds."""
+
+    @abstractmethod
+    async def delete(self, key: str) -> None:
+        """Delete object at key."""