Add per-calendar reminders_enabled flag (server)

New boolean column on every calendar type (CalDAV, local, Google, Home
Assistant, iCal subscription), default ON so existing data is unchanged.
Lets a calendar be shown without generating reminders/notifications.

- models.py: reminders_enabled on the five calendar models
- main.py _migrate(): ALTER TABLE ADD COLUMN for each table
- caldav/google/homeassistant/local/ical routers: expose the flag in the
  calendar serialization and accept it in the calendar/subscription update
  endpoints

Clients map event -> (source, calendar_id) -> reminders_enabled to skip
scheduling notifications for disabled calendars.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/routers/ical_router.py

@@ -29,6 +29,7 @@ class SubscriptionUpdate(BaseModel):
     color: Optional[str] = None
     enabled: Optional[bool] = None
     refresh_minutes: Optional[int] = None
+    reminders_enabled: Optional[bool] = None
 
 
 def _sub_dict(sub: models.ICalSubscription) -> dict:
@@ -38,6 +39,7 @@ def _sub_dict(sub: models.ICalSubscription) -> dict:
         "url": sub.url,
         "color": sub.color,
         "enabled": sub.enabled,
+        "reminders_enabled": bool(sub.reminders_enabled),
         "refresh_minutes": sub.refresh_minutes,
         "last_fetched": sub.last_fetched.isoformat() if sub.last_fetched else None,
     }
@@ -215,6 +217,8 @@ def update_subscription(
         sub.enabled = data.enabled
     if data.refresh_minutes is not None:
         sub.refresh_minutes = data.refresh_minutes
+    if data.reminders_enabled is not None:
+        sub.reminders_enabled = data.reminders_enabled
     db.commit()
     return {"ok": True}