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/cryptography/exceptions.py

@@ -0,0 +1,52 @@
+# This file is dual licensed under the terms of the Apache License, Version
+# 2.0, and the BSD License. See the LICENSE file in the root of this repository
+# for complete details.
+
+from __future__ import annotations
+
+import typing
+
+from cryptography.hazmat.bindings._rust import exceptions as rust_exceptions
+
+if typing.TYPE_CHECKING:
+    from cryptography.hazmat.bindings._rust import openssl as rust_openssl
+
+_Reasons = rust_exceptions._Reasons
+
+
+class UnsupportedAlgorithm(Exception):
+    def __init__(self, message: str, reason: _Reasons | None = None) -> None:
+        super().__init__(message)
+        self._reason = reason
+
+
+class AlreadyFinalized(Exception):
+    pass
+
+
+class AlreadyUpdated(Exception):
+    pass
+
+
+class NotYetFinalized(Exception):
+    pass
+
+
+class InvalidTag(Exception):
+    pass
+
+
+class InvalidSignature(Exception):
+    pass
+
+
+class InternalError(Exception):
+    def __init__(
+        self, msg: str, err_code: list[rust_openssl.OpenSSLError]
+    ) -> None:
+        super().__init__(msg)
+        self.err_code = err_code
+
+
+class InvalidKey(Exception):
+    pass