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/main.py

@@ -201,6 +201,15 @@ def _migrate():
             logging.info("Migration: added color to group_members")
         except Exception:
             pass
+        # Per-calendar reminder toggle (default ON = unchanged behaviour for existing data).
+        for tbl in ("calendars", "local_calendars", "google_calendars",
+                    "homeassistant_calendars", "ical_subscriptions"):
+            try:
+                conn.execute(text(f"ALTER TABLE {tbl} ADD COLUMN reminders_enabled BOOLEAN DEFAULT 1"))
+                conn.commit()
+                logging.info("Migration: added reminders_enabled to %s", tbl)
+            except Exception:
+                pass
 
 _migrate()