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/triggers/base.py

@@ -0,0 +1,35 @@
+import random
+from abc import ABCMeta, abstractmethod
+from datetime import timedelta
+
+
+class BaseTrigger(metaclass=ABCMeta):
+    """Abstract base class that defines the interface that every trigger must implement."""
+
+    __slots__ = ()
+
+    @abstractmethod
+    def get_next_fire_time(self, previous_fire_time, now):
+        """
+        Returns the next datetime to fire on, If no such datetime can be calculated, returns
+        ``None``.
+
+        :param datetime.datetime previous_fire_time: the previous time the trigger was fired
+        :param datetime.datetime now: current datetime
+        """
+
+    def _apply_jitter(self, next_fire_time, jitter, now):
+        """
+        Randomize ``next_fire_time`` by adding a random value (the jitter).
+
+        :param datetime.datetime|None next_fire_time: next fire time without jitter applied. If
+            ``None``, returns ``None``.
+        :param int|None jitter: maximum number of seconds to add to ``next_fire_time``
+            (if ``None`` or ``0``, returns ``next_fire_time``)
+        :param datetime.datetime now: current datetime
+        :return datetime.datetime|None: next fire time with a jitter.
+        """
+        if next_fire_time is None or not jitter:
+            return next_fire_time
+
+        return next_fire_time + timedelta(seconds=random.uniform(0, jitter))