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/rsa/asn1.py

@@ -0,0 +1,52 @@
+#  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+"""ASN.1 definitions.
+
+Not all ASN.1-handling code use these definitions, but when it does, they should be here.
+"""
+
+from pyasn1.type import univ, namedtype, tag
+
+
+class PubKeyHeader(univ.Sequence):
+    componentType = namedtype.NamedTypes(
+        namedtype.NamedType("oid", univ.ObjectIdentifier()),
+        namedtype.NamedType("parameters", univ.Null()),
+    )
+
+
+class OpenSSLPubKey(univ.Sequence):
+    componentType = namedtype.NamedTypes(
+        namedtype.NamedType("header", PubKeyHeader()),
+        # This little hack (the implicit tag) allows us to get a Bit String as Octet String
+        namedtype.NamedType(
+            "key",
+            univ.OctetString().subtype(implicitTag=tag.Tag(tagClass=0, tagFormat=0, tagId=3)),
+        ),
+    )
+
+
+class AsnPubKey(univ.Sequence):
+    """ASN.1 contents of DER encoded public key:
+
+    RSAPublicKey ::= SEQUENCE {
+         modulus           INTEGER,  -- n
+         publicExponent    INTEGER,  -- e
+    """
+
+    componentType = namedtype.NamedTypes(
+        namedtype.NamedType("modulus", univ.Integer()),
+        namedtype.NamedType("publicExponent", univ.Integer()),
+    )