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/py.py

@@ -0,0 +1,15 @@
+# shim for pylib going away
+# if pylib is installed this file will get skipped
+# (`py/__init__.py` has higher precedence)
+from __future__ import annotations
+
+import sys
+
+import _pytest._py.error as error
+import _pytest._py.path as path
+
+
+sys.modules["py.error"] = error
+sys.modules["py.path"] = path
+
+__all__ = ["error", "path"]