Remove grace period logic and improve meeting deactivation

- Removed grace_period_minutes and last_participant_left_at fields
- Simplified deactivation logic based on actual usage patterns:
  * Active sessions: Keep meeting active regardless of scheduled time
  * Calendar meetings: Wait until scheduled end if unused, deactivate immediately once used and empty
  * On-the-fly meetings: Deactivate immediately when empty
- Created migration to drop unused database columns
- Updated tests to remove grace period test cases

www/app/reflector-api.d.ts

@@ -1072,13 +1072,6 @@ export interface components {
       calendar_metadata?: {
         [key: string]: unknown;
       } | null;
-      /** Last Participant Left At */
-      last_participant_left_at?: string | null;
-      /**
-       * Grace Period Minutes
-       * @default 15
-       */
-      grace_period_minutes: number;
     };
     /** MeetingConsentRequest */
     MeetingConsentRequest: {