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

app/plugins/builtins/tasks/jobs.py

@@ -0,0 +1,51 @@
+"""ARQ reminder job for due tasks — sends notifications for overdue/due tasks."""
+
+from __future__ import annotations
+
+import logging
+from datetime import datetime, timezone
+
+from app.core.db import get_session_factory
+
+logger = logging.getLogger(__name__)
+
+
+async def tasks_due_reminder(ctx: dict) -> None:
+    """Check for due tasks and send notifications to assigned users.
+
+    Runs daily at 8:00 via cron. Finds all non-done tasks with due_date <= now
+    and creates a notification for the assigned user.
+    """
+    from app.plugins.builtins.tasks.services import get_due_tasks
+    from app.models.tenant import Tenant
+    from app.core.notifications import create_notification
+    from sqlalchemy import select
+
+    factory = get_session_factory()
+    async with factory() as db:
+        result = await db.execute(select(Tenant))
+        tenants = result.scalars().all()
+
+        total_notified = 0
+        for tenant in tenants:
+            try:
+                due_tasks = await get_due_tasks(db, tenant.id)
+                for task in due_tasks:
+                    if not task.get("assigned_to"):
+                        continue
+                    await create_notification(
+                        db,
+                        user_id=task["assigned_to"],
+                        tenant_id=str(tenant.id),
+                        type="task_due",
+                        title=f"Task due: {task['title']}",
+                        body=f"Task '{task['title']}' is due. Priority: {task['priority']}",
+                        data={"task_id": task["id"], "due_date": task.get("due_date")},
+                    )
+                    total_notified += 1
+            except Exception:
+                logger.warning(f"Failed to process due tasks for tenant {tenant.id}", exc_info=True)
+
+        if total_notified > 0:
+            logger.info(f"Tasks reminder: sent {total_notified} notifications")
+        await db.commit()