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/starlette/background.py

@@ -0,0 +1,36 @@
+from __future__ import annotations
+
+from collections.abc import Callable, Sequence
+from typing import Any, ParamSpec
+
+from starlette._utils import is_async_callable
+from starlette.concurrency import run_in_threadpool
+
+P = ParamSpec("P")
+
+
+class BackgroundTask:
+    def __init__(self, func: Callable[P, Any], *args: P.args, **kwargs: P.kwargs) -> None:
+        self.func = func
+        self.args = args
+        self.kwargs = kwargs
+        self.is_async = is_async_callable(func)
+
+    async def __call__(self) -> None:
+        if self.is_async:
+            await self.func(*self.args, **self.kwargs)
+        else:
+            await run_in_threadpool(self.func, *self.args, **self.kwargs)
+
+
+class BackgroundTasks(BackgroundTask):
+    def __init__(self, tasks: Sequence[BackgroundTask] | None = None):
+        self.tasks = list(tasks) if tasks else []
+
+    def add_task(self, func: Callable[P, Any], *args: P.args, **kwargs: P.kwargs) -> None:
+        task = BackgroundTask(func, *args, **kwargs)
+        self.tasks.append(task)
+
+    async def __call__(self) -> None:
+        for task in self.tasks:
+            await task()