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/local_router.py

@@ -30,6 +30,7 @@ class CalendarUpdate(BaseModel):
     name: Optional[str] = None
     color: Optional[str] = None
     enabled: Optional[bool] = None
+    reminders_enabled: Optional[bool] = None
 
 
 class EventCreate(BaseModel):
@@ -72,6 +73,7 @@ def _cal_dict(cal: models.LocalCalendar, *, owned: bool = True,
         "name": cal.name,
         "color": cal.color,
         "enabled": cal.enabled,
+        "reminders_enabled": bool(cal.reminders_enabled),
         "type": "local",
         "owned": owned,
     }
@@ -199,6 +201,8 @@ def update_calendar(
         cal.color = data.color
     if data.enabled is not None:
         cal.enabled = data.enabled
+    if data.reminders_enabled is not None:
+        cal.reminders_enabled = data.reminders_enabled
     db.commit()
     return {"ok": True}