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/apscheduler/schedulers/background.py

@@ -0,0 +1,42 @@
+from threading import Event, Thread
+
+from apscheduler.schedulers.base import BaseScheduler
+from apscheduler.schedulers.blocking import BlockingScheduler
+from apscheduler.util import asbool
+
+
+class BackgroundScheduler(BlockingScheduler):
+    """
+    A scheduler that runs in the background using a separate thread
+    (:meth:`~apscheduler.schedulers.base.BaseScheduler.start` will return immediately).
+
+    Extra options:
+
+    ========== =============================================================================
+    ``daemon`` Set the ``daemon`` option in the background thread (defaults to ``True``, see
+               `the documentation
+               <https://docs.python.org/3.4/library/threading.html#thread-objects>`_
+               for further details)
+    ========== =============================================================================
+    """
+
+    _thread = None
+
+    def _configure(self, config):
+        self._daemon = asbool(config.pop("daemon", True))
+        super()._configure(config)
+
+    def start(self, *args, **kwargs):
+        if self._event is None or self._event.is_set():
+            self._event = Event()
+
+        BaseScheduler.start(self, *args, **kwargs)
+        self._thread = Thread(
+            target=self._main_loop, name="APScheduler", daemon=self._daemon
+        )
+        self._thread.start()
+
+    def shutdown(self, *args, **kwargs):
+        super().shutdown(*args, **kwargs)
+        self._thread.join()
+        del self._thread