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/rfc6187.py

@@ -0,0 +1,22 @@
+#
+# 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
+#
+# X.509v3 Certificates for Secure Shell Authentication
+#
+# ASN.1 source from:
+# https://www.rfc-editor.org/rfc/rfc6187.txt
+#
+
+from pyasn1.type import univ
+
+id_pkix = univ.ObjectIdentifier('1.3.6.1.5.5.7')
+
+id_kp = id_pkix + (3, )
+
+id_kp_secureShellClient = id_kp + (21, )
+id_kp_secureShellServer = id_kp + (22, )