feat: Add event automation system for plugins attached to appointments

- Add GlobalEventPlugin model for auto-attaching plugins to all events
- Create signals for auto-attachment on new events and rescheduling
- Add API endpoints for global event plugins (CRUD, toggle, reapply)
- Update CreateTaskModal with "Scheduled Task" vs "Event Automation" choice
- Add option to apply to all events or future events only
- Display event automations in Tasks page alongside scheduled tasks
- Add EditEventAutomationModal for editing trigger and timing
- Handle event reschedule - update Celery task timing on time/duration changes
- Add Marketplace to Plugins menu in sidebar

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/src/i18n/locales/en.json

@@ -74,6 +74,7 @@
   "nav": {
     "dashboard": "Dashboard",
     "scheduler": "Scheduler",
+    "tasks": "Tasks",
     "customers": "Customers",
     "resources": "Resources",
     "services": "Services",