[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/validation.py

@@ -0,0 +1,21 @@
+ACCEPTED_MIME_TYPES = frozenset(["image/jpeg", "image/png", "image/gif", "image/webp"])
+
+
+class MimeTypeError(ValueError):
+    pass
+
+
+class FileSizeError(ValueError):
+    pass
+
+
+def validate_mime_type(mime_type: str) -> None:
+    if mime_type not in ACCEPTED_MIME_TYPES:
+        raise MimeTypeError(f"Unsupported MIME type: {mime_type}")
+
+
+def validate_file_size(size_bytes: int, max_bytes: int) -> None:
+    if size_bytes <= 0:
+        raise FileSizeError("File must not be empty")
+    if size_bytes > max_bytes:
+        raise FileSizeError(f"File size {size_bytes} exceeds limit of {max_bytes} bytes")