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_modules/rfc6120.py

@@ -0,0 +1,43 @@
+#
+# This file is part of pyasn1-modules software.
+#
+# Created by Russ Housley.
+#
+# Copyright (c) 2019, Vigil Security, LLC
+# License: http://snmplabs.com/pyasn1/license.html
+#
+# Extensible Messaging and Presence Protocol (XMPP)
+#
+# ASN.1 source from:
+# https://www.rfc-editor.org/rfc/rfc6120.txt
+#
+
+from pyasn1.type import char
+from pyasn1.type import univ
+
+from pyasn1_modules import rfc5280
+
+MAX = float('inf')
+
+
+# XmppAddr Identifier Type as specified in Section 13.7.1.4. of RFC 6120
+
+id_pkix = rfc5280.id_pkix
+
+id_on = id_pkix + (8, )
+
+id_on_xmppAddr = id_on + (5, )
+
+
+class XmppAddr(char.UTF8String):
+    pass
+
+
+# Map of Other Name OIDs to Other Name is added to the
+# ones that are in rfc5280.py
+
+_anotherNameMapUpdate = {
+    id_on_xmppAddr: XmppAddr(),
+}
+
+rfc5280.anotherNameMap.update(_anotherNameMapUpdate)