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/rfc5208.py

@@ -0,0 +1,56 @@
+#
+# This file is part of pyasn1-modules software.
+#
+# Copyright (c) 2005-2020, Ilya Etingof <etingof@gmail.com>
+# License: http://snmplabs.com/pyasn1/license.html
+#
+# PKCS#8 syntax
+#
+# ASN.1 source from:
+# http://tools.ietf.org/html/rfc5208
+#
+# Sample captures could be obtained with "openssl pkcs8 -topk8" command
+#
+from pyasn1_modules import rfc2251
+from pyasn1_modules.rfc2459 import *
+
+
+class KeyEncryptionAlgorithms(AlgorithmIdentifier):
+    pass
+
+
+class PrivateKeyAlgorithms(AlgorithmIdentifier):
+    pass
+
+
+class EncryptedData(univ.OctetString):
+    pass
+
+
+class EncryptedPrivateKeyInfo(univ.Sequence):
+    componentType = namedtype.NamedTypes(
+        namedtype.NamedType('encryptionAlgorithm', AlgorithmIdentifier()),
+        namedtype.NamedType('encryptedData', EncryptedData())
+    )
+
+
+class PrivateKey(univ.OctetString):
+    pass
+
+
+class Attributes(univ.SetOf):
+    componentType = rfc2251.Attribute()
+
+
+class Version(univ.Integer):
+    namedValues = namedval.NamedValues(('v1', 0), ('v2', 1))
+
+
+class PrivateKeyInfo(univ.Sequence):
+    componentType = namedtype.NamedTypes(
+        namedtype.NamedType('version', Version()),
+        namedtype.NamedType('privateKeyAlgorithm', AlgorithmIdentifier()),
+        namedtype.NamedType('privateKey', PrivateKey()),
+        namedtype.OptionalNamedType('attributes', Attributes().subtype(
+            implicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatConstructed, 0)))
+    )