feat(5.21): Tasks Plugin — activities with status, priority, due dates, ARQ reminders

- New builtin plugin app/plugins/builtins/tasks/ with full CRUD
- Task model with TenantMixin (title, description, status, priority, due_date, assigned_to, contact_id)
- Routes: GET/POST /tasks, GET/PATCH/DELETE /tasks/{id}, POST /tasks/{id}/assign, POST /tasks/{id}/status
- All routes RBAC-protected (tasks:read, tasks:write, tasks:delete)
- ARQ cron job tasks_due_reminder (daily 8:00) sends notifications for due tasks
- Migration 0001_initial.sql creates tasks table with indexes
- Frontend: Tasks.tsx page with list, filter, create/edit modal, detail modal
- Frontend: api/tasks.ts with React Query hooks
- Route /tasks in routes/index.tsx, sidebar entry via plugin manifest
- i18n keys for nav.tasks and tasks.* in de.json and en.json
- Tests: test_tasks.py (11 tests) + Tasks.test.tsx (3 tests)
- Registered TasksPlugin in conftest.py and worker.py

frontend/src/routes/index.tsx

@@ -40,6 +40,7 @@ const AutomationDashboardPage = React.lazy(() => import('@/pages/AutomationDashb
 const AgentDashboardPage = React.lazy(() => import('@/pages/AgentDashboard').then(m => ({ default: m.AgentDashboardPage })));
 const AutomationSettingsPage = React.lazy(() => import('@/pages/AutomationSettings').then(m => ({ default: m.AutomationSettingsPage })));
 const ReportsPage = React.lazy(() => import('@/pages/Reports').then(m => ({ default: m.ReportsPage })));
+const TasksPage = React.lazy(() => import('@/pages/Tasks').then(m => ({ default: m.TasksPage })));
 
 /** Centered spinner fallback for lazy-loaded routes */
 function PageLoader() {
@@ -91,6 +92,7 @@ const router = createBrowserRouter([
       { path: '/automation', element: withSuspense(<AutomationDashboardPage />) },
       { path: '/agents', element: withSuspense(<AgentDashboardPage />) },
       { path: '/reports', element: withSuspense(<ReportsPage />) },
+      { path: '/tasks', element: withSuspense(<TasksPage />) },
       { path: '/profile', element: withSuspense(<SettingsProfilePage />) },
       {
         path: '/settings',