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/gevent.py

@@ -0,0 +1,34 @@
+from apscheduler.schedulers.base import BaseScheduler
+from apscheduler.schedulers.blocking import BlockingScheduler
+
+try:
+    import gevent
+    from gevent.event import Event
+    from gevent.lock import RLock
+except ImportError as exc:  # pragma: nocover
+    raise ImportError("GeventScheduler requires gevent installed") from exc
+
+
+class GeventScheduler(BlockingScheduler):
+    """A scheduler that runs as a Gevent greenlet."""
+
+    _greenlet = None
+
+    def start(self, *args, **kwargs):
+        self._event = Event()
+        BaseScheduler.start(self, *args, **kwargs)
+        self._greenlet = gevent.spawn(self._main_loop)
+        return self._greenlet
+
+    def shutdown(self, *args, **kwargs):
+        super().shutdown(*args, **kwargs)
+        self._greenlet.join()
+        del self._greenlet
+
+    def _create_lock(self):
+        return RLock()
+
+    def _create_default_executor(self):
+        from apscheduler.executors.gevent import GeventExecutor
+
+        return GeventExecutor()