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/PIL/_imaging.pyi

@@ -0,0 +1,31 @@
+from typing import Any
+
+class ImagingCore:
+    def __getitem__(self, index: int) -> float | tuple[int, ...] | None: ...
+    def __getattr__(self, name: str) -> Any: ...
+
+class ImagingFont:
+    def __getattr__(self, name: str) -> Any: ...
+
+class ImagingDraw:
+    def __getattr__(self, name: str) -> Any: ...
+
+class PixelAccess:
+    def __getitem__(self, xy: tuple[int, int]) -> float | tuple[int, ...]: ...
+    def __setitem__(
+        self, xy: tuple[int, int], color: float | tuple[int, ...]
+    ) -> None: ...
+
+class ImagingDecoder:
+    def __getattr__(self, name: str) -> Any: ...
+
+class ImagingEncoder:
+    def __getattr__(self, name: str) -> Any: ...
+
+class _Outline:
+    def close(self) -> None: ...
+    def __getattr__(self, name: str) -> Any: ...
+
+def font(image: ImagingCore, glyphdata: bytes) -> ImagingFont: ...
+def outline() -> _Outline: ...
+def __getattr__(name: str) -> Any: ...