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/sniffio/_tests/test_sniffio.py

@@ -0,0 +1,84 @@
+import os
+import sys
+
+import pytest
+
+from .. import (
+    current_async_library, AsyncLibraryNotFoundError,
+    current_async_library_cvar, thread_local
+)
+
+
+def test_basics_cvar():
+    with pytest.raises(AsyncLibraryNotFoundError):
+        current_async_library()
+
+    token = current_async_library_cvar.set("generic-lib")
+    try:
+        assert current_async_library() == "generic-lib"
+    finally:
+        current_async_library_cvar.reset(token)
+
+    with pytest.raises(AsyncLibraryNotFoundError):
+        current_async_library()
+
+
+def test_basics_tlocal():
+    with pytest.raises(AsyncLibraryNotFoundError):
+        current_async_library()
+
+    old_name, thread_local.name = thread_local.name, "generic-lib"
+    try:
+        assert current_async_library() == "generic-lib"
+    finally:
+        thread_local.name = old_name
+
+    with pytest.raises(AsyncLibraryNotFoundError):
+        current_async_library()
+
+
+def test_asyncio():
+    import asyncio
+
+    with pytest.raises(AsyncLibraryNotFoundError):
+        current_async_library()
+
+    ran = []
+
+    async def this_is_asyncio():
+        assert current_async_library() == "asyncio"
+        # Call it a second time to exercise the caching logic
+        assert current_async_library() == "asyncio"
+        ran.append(True)
+
+    asyncio.run(this_is_asyncio())
+    assert ran == [True]
+
+    with pytest.raises(AsyncLibraryNotFoundError):
+        current_async_library()
+
+
+@pytest.mark.skipif(
+    sys.version_info >= (3, 12),
+    reason=
+    "curio broken on 3.12 (https://github.com/python-trio/sniffio/pull/42)",
+)
+def test_curio():
+    import curio
+
+    with pytest.raises(AsyncLibraryNotFoundError):
+        current_async_library()
+
+    ran = []
+
+    async def this_is_curio():
+        assert current_async_library() == "curio"
+        # Call it a second time to exercise the caching logic
+        assert current_async_library() == "curio"
+        ran.append(True)
+
+    curio.run(this_is_curio)
+    assert ran == [True]
+
+    with pytest.raises(AsyncLibraryNotFoundError):
+        current_async_library()