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/rfc3414.py

@@ -0,0 +1,28 @@
+#
+# This file is part of pyasn1-modules software.
+#
+# Copyright (c) 2005-2020, Ilya Etingof <etingof@gmail.com>
+# License: http://snmplabs.com/pyasn1/license.html
+#
+# SNMPv3 message syntax
+#
+# ASN.1 source from:
+# http://www.ietf.org/rfc/rfc3414.txt
+#
+from pyasn1.type import constraint
+from pyasn1.type import namedtype
+from pyasn1.type import univ
+
+
+class UsmSecurityParameters(univ.Sequence):
+    componentType = namedtype.NamedTypes(
+        namedtype.NamedType('msgAuthoritativeEngineID', univ.OctetString()),
+        namedtype.NamedType('msgAuthoritativeEngineBoots',
+                            univ.Integer().subtype(subtypeSpec=constraint.ValueRangeConstraint(0, 2147483647))),
+        namedtype.NamedType('msgAuthoritativeEngineTime',
+                            univ.Integer().subtype(subtypeSpec=constraint.ValueRangeConstraint(0, 2147483647))),
+        namedtype.NamedType('msgUserName',
+                            univ.OctetString().subtype(subtypeSpec=constraint.ValueSizeConstraint(0, 32))),
+        namedtype.NamedType('msgAuthenticationParameters', univ.OctetString()),
+        namedtype.NamedType('msgPrivacyParameters', univ.OctetString())
+    )