feat(phase1): add image storage utilities

- Create backend/services/image_storage.py with 4 core functions:
  - sanitize_filename(): remove unsafe chars, limit to 255 chars, convert to lowercase
  - get_unique_filename(): handle collisions with UUID suffix (format: {name}_{uuid8}_{variant}.jpg)
  - ensure_image_directories(): create /images/ root and category subdirs on startup
  - save_image(): save bytes to /images/{category}/{filename}, returns relative path
- Create comprehensive test suite (22 tests) covering all functionality
- Integrate ensure_image_directories() into FastAPI startup event
- Directory structure: /images/{category}/{filename}
- Collision handling: auto-suffix with UUID if filename exists
- All tests passing, pathlib.Path for safe operations

venv/lib/python3.12/site-packages/fastapi/cli.py

@@ -0,0 +1,13 @@
+try:
+    from fastapi_cli.cli import main as cli_main
+
+except ImportError:  # pragma: no cover
+    cli_main = None  # type: ignore
+
+
+def main() -> None:
+    if not cli_main:  # type: ignore[truthy-function]  # ty: ignore[unused-ignore-comment]
+        message = 'To use the fastapi command, please install "fastapi[standard]":\n\n\tpip install "fastapi[standard]"\n'
+        print(message)
+        raise RuntimeError(message)  # noqa: B904
+    cli_main()