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/includes/python.pxd

@@ -0,0 +1,31 @@
+cdef extern from "Python.h":
+    int PY_VERSION_HEX
+
+    unicode PyUnicode_FromString(const char *)
+
+    void* PyMem_RawMalloc(size_t n) nogil
+    void* PyMem_RawRealloc(void *p, size_t n) nogil
+    void* PyMem_RawCalloc(size_t nelem, size_t elsize) nogil
+    void PyMem_RawFree(void *p) nogil
+
+    object PyUnicode_EncodeFSDefault(object)
+    void PyErr_SetInterrupt() nogil
+
+    object PyMemoryView_FromMemory(char *mem, ssize_t size, int flags)
+    object PyMemoryView_FromObject(object obj)
+    int PyMemoryView_Check(object obj)
+
+    cdef enum:
+        PyBUF_WRITE
+
+
+cdef extern from "includes/compat.h":
+    object Context_CopyCurrent()
+    int Context_Enter(object) except -1
+    int Context_Exit(object) except -1
+
+    void PyOS_BeforeFork()
+    void PyOS_AfterFork_Parent()
+    void PyOS_AfterFork_Child()
+
+    void _Py_RestoreSignals()