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/flowcontrol.pxd

@@ -0,0 +1,23 @@
+# flake8: noqa
+
+
+cdef inline add_flowcontrol_defaults(high, low, int kb):
+    cdef int h, l
+    if high is None:
+        if low is None:
+            h = kb * 1024
+        else:
+            l = low
+            h = 4 * l
+    else:
+        h = high
+    if low is None:
+        l = h // 4
+    else:
+        l = low
+
+    if not h >= l >= 0:
+        raise ValueError('high (%r) must be >= low (%r) must be >= 0' %
+                         (h, l))
+
+    return h, l