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/iniconfig/exceptions.py

@@ -0,0 +1,16 @@
+from typing import Final
+
+
+class ParseError(Exception):
+    path: Final[str]
+    lineno: Final[int]
+    msg: Final[str]
+
+    def __init__(self, path: str, lineno: int, msg: str) -> None:
+        super().__init__(path, lineno, msg)
+        self.path = path
+        self.lineno = lineno
+        self.msg = msg
+
+    def __str__(self) -> str:
+        return f"{self.path}:{self.lineno + 1}: {self.msg}"