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/rfc4387.py

@@ -0,0 +1,23 @@
+#
+# 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
+#
+# Certificate Store Access via HTTP
+#
+# ASN.1 source from:
+# https://www.rfc-editor.org/rfc/rfc4387.txt
+#
+
+
+from pyasn1.type import univ
+
+
+id_ad = univ.ObjectIdentifier((1, 3, 6, 1, 5, 5, 7, 48, ))
+
+id_ad_http_certs = id_ad + (6, )
+
+id_ad_http_crls = id_ad  + (7,)