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/poll.pxd

@@ -0,0 +1,25 @@
+cdef class UVPoll(UVHandle):
+    cdef:
+        int fd
+        Handle reading_handle
+        Handle writing_handle
+
+    cdef _init(self, Loop loop, int fd)
+    cdef _close(self)
+
+    cdef inline _poll_start(self, int flags)
+    cdef inline _poll_stop(self)
+
+    cdef int is_active(self) noexcept
+
+    cdef is_reading(self)
+    cdef is_writing(self)
+
+    cdef start_reading(self, Handle callback)
+    cdef start_writing(self, Handle callback)
+    cdef stop_reading(self)
+    cdef stop_writing(self)
+    cdef stop(self)
+
+    @staticmethod
+    cdef UVPoll new(Loop loop, int fd)