feat(calendar): update Event model and patch caldav.create_event with uid param

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/services/caldav.py

@@ -176,6 +176,7 @@ async def create_event(
     reminder_minutes: int | None = None,
     attendees: list[str] | None = None,
     calendar_name: str | None = None,
+    uid: str | None = None,
 ) -> dict:
     """Create a calendar event.
 
@@ -193,6 +194,11 @@ async def create_event(
     cal.add("version", "2.0")
 
     event = icalendar.Event()
+    if uid:
+        # Remove auto-generated UID if the library added one, then inject ours
+        if "UID" in event:
+            del event["UID"]
+        event.add("uid", uid)
     event.add("summary", title)
 
     if all_day: