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/starlette/exceptions.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+import http
+from collections.abc import Mapping
+
+
+class HTTPException(Exception):
+    def __init__(self, status_code: int, detail: str | None = None, headers: Mapping[str, str] | None = None) -> None:
+        if detail is None:
+            detail = http.HTTPStatus(status_code).phrase
+        self.status_code = status_code
+        self.detail = detail
+        self.headers = headers
+
+    def __str__(self) -> str:
+        return f"{self.status_code}: {self.detail}"
+
+    def __repr__(self) -> str:
+        class_name = self.__class__.__name__
+        return f"{class_name}(status_code={self.status_code!r}, detail={self.detail!r})"
+
+
+class WebSocketException(Exception):
+    def __init__(self, code: int, reason: str | None = None) -> None:
+        self.code = code
+        self.reason = reason or ""
+
+    def __str__(self) -> str:
+        return f"{self.code}: {self.reason}"
+
+    def __repr__(self) -> str:
+        class_name = self.__class__.__name__
+        return f"{class_name}(code={self.code!r}, reason={self.reason!r})"