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/pyasn1/compat/integer.py

@@ -0,0 +1,13 @@
+#
+# This file is part of pyasn1 software.
+#
+# Copyright (c) 2005-2020, Ilya Etingof <etingof@gmail.com>
+# License: https://pyasn1.readthedocs.io/en/latest/license.html
+#
+def to_bytes(value, signed=False, length=0):
+    length = max(value.bit_length(), length)
+
+    if signed and length % 8 == 0:
+        length += 1
+
+    return value.to_bytes(length // 8 + (length % 8 and 1 or 0), 'big', signed=signed)