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/uvloop/handles/tcp.pxd

@@ -0,0 +1,26 @@
+cdef class TCPServer(UVStreamServer):
+    cdef bind(self, system.sockaddr* addr, unsigned int flags=*)
+
+    @staticmethod
+    cdef TCPServer new(Loop loop, object protocol_factory, Server server,
+                       unsigned int flags,
+                       object backlog,
+                       object ssl,
+                       object ssl_handshake_timeout,
+                       object ssl_shutdown_timeout)
+
+
+cdef class TCPTransport(UVStream):
+    cdef:
+        bint __peername_set
+        bint __sockname_set
+        system.sockaddr_storage __peername
+        system.sockaddr_storage __sockname
+
+    cdef bind(self, system.sockaddr* addr, unsigned int flags=*)
+    cdef connect(self, system.sockaddr* addr)
+    cdef _set_nodelay(self)
+
+    @staticmethod
+    cdef TCPTransport new(Loop loop, object protocol, Server server,
+                          object waiter, object context)