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/slowapi/errors.py

@@ -0,0 +1,27 @@
+"""
+errors and exceptions
+"""
+
+from starlette.exceptions import HTTPException
+
+from .wrappers import Limit
+
+
+class RateLimitExceeded(HTTPException):
+    """
+    exception raised when a rate limit is hit.
+    """
+
+    limit = None
+
+    def __init__(self, limit: Limit) -> None:
+        self.limit = limit
+        if limit.error_message:
+            description: str = (
+                limit.error_message
+                if not callable(limit.error_message)
+                else limit.error_message()
+            )
+        else:
+            description = str(limit.limit)
+        super(RateLimitExceeded, self).__init__(status_code=429, detail=description)