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/rfc7773.py

@@ -0,0 +1,52 @@
+#
+# This file is part of pyasn1-modules software.
+#
+# Created by Russ Housley with some assistance from asn1ate v.0.6.0.
+#
+# Copyright (c) 2019, Vigil Security, LLC
+# License: http://snmplabs.com/pyasn1/license.html
+#
+# Authentication Context Certificate Extension
+#
+# ASN.1 source from:
+# https://www.rfc-editor.org/rfc/rfc7773.txt
+#
+
+from pyasn1.type import char
+from pyasn1.type import constraint
+from pyasn1.type import namedtype
+from pyasn1.type import univ
+
+from pyasn1_modules import rfc5280
+
+MAX = float('inf')
+
+
+# Authentication Context Extension
+
+e_legnamnden = univ.ObjectIdentifier('1.2.752.201')
+
+id_eleg_ce = e_legnamnden + (5, )
+
+id_ce_authContext = id_eleg_ce + (1, )
+
+
+class AuthenticationContext(univ.Sequence):
+    componentType = namedtype.NamedTypes(
+        namedtype.NamedType('contextType', char.UTF8String()),
+        namedtype.OptionalNamedType('contextInfo', char.UTF8String())
+    )
+
+class AuthenticationContexts(univ.SequenceOf):
+    componentType = AuthenticationContext()
+    subtypeSpec=constraint.ValueSizeConstraint(1, MAX)
+
+
+# Map of Certificate Extension OIDs to Extensions added to the
+# ones that are in rfc5280.py
+
+_certificateExtensionsMapUpdate = {
+    id_ce_authContext: AuthenticationContexts(),
+}
+
+rfc5280.certificateExtensionsMap.update(_certificateExtensionsMapUpdate)