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/rfc3560.py

@@ -0,0 +1,74 @@
+#
+# 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
+#
+# RSAES-OAEP Key Transport Algorithm in CMS
+#
+# Notice that all of the things needed in RFC 3560 are also defined
+# in RFC 4055.  So, they are all pulled from the RFC 4055 module into
+# this one so that people looking a RFC 3560 can easily find them.
+#
+# ASN.1 source from:
+# https://www.rfc-editor.org/rfc/rfc3560.txt
+#
+
+from pyasn1_modules import rfc4055
+
+id_sha1 = rfc4055.id_sha1
+
+id_sha256 = rfc4055.id_sha256
+
+id_sha384 = rfc4055.id_sha384
+
+id_sha512 = rfc4055.id_sha512
+
+id_mgf1 = rfc4055.id_mgf1
+
+rsaEncryption = rfc4055.rsaEncryption
+
+id_RSAES_OAEP = rfc4055.id_RSAES_OAEP
+
+id_pSpecified = rfc4055.id_pSpecified
+
+sha1Identifier = rfc4055.sha1Identifier
+
+sha256Identifier = rfc4055.sha256Identifier
+
+sha384Identifier = rfc4055.sha384Identifier
+
+sha512Identifier = rfc4055.sha512Identifier
+
+mgf1SHA1Identifier = rfc4055.mgf1SHA1Identifier
+
+mgf1SHA256Identifier = rfc4055.mgf1SHA256Identifier
+
+mgf1SHA384Identifier = rfc4055.mgf1SHA384Identifier
+
+mgf1SHA512Identifier = rfc4055.mgf1SHA512Identifier
+
+pSpecifiedEmptyIdentifier = rfc4055.pSpecifiedEmptyIdentifier
+
+
+class RSAES_OAEP_params(rfc4055.RSAES_OAEP_params):
+    pass
+
+
+rSAES_OAEP_Default_Params = RSAES_OAEP_params()
+
+rSAES_OAEP_Default_Identifier = rfc4055.rSAES_OAEP_Default_Identifier
+
+rSAES_OAEP_SHA256_Params = rfc4055.rSAES_OAEP_SHA256_Params
+
+rSAES_OAEP_SHA256_Identifier = rfc4055.rSAES_OAEP_SHA256_Identifier
+
+rSAES_OAEP_SHA384_Params = rfc4055.rSAES_OAEP_SHA384_Params
+
+rSAES_OAEP_SHA384_Identifier = rfc4055.rSAES_OAEP_SHA384_Identifier
+
+rSAES_OAEP_SHA512_Params = rfc4055.rSAES_OAEP_SHA512_Params
+
+rSAES_OAEP_SHA512_Identifier = rfc4055.rSAES_OAEP_SHA512_Identifier