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/plugin.py

@@ -0,0 +1,65 @@
+"""Tasks plugin — manage free tasks/activities linked to contacts."""
+
+from __future__ import annotations
+
+from app.plugins.base import BasePlugin
+from app.plugins.manifest import (
+    PluginManifest,
+    PluginRouteDef,
+    FrontendMenuItem,
+    FrontendPageRoute,
+    CronJobContribution,
+)
+
+
+class TasksPlugin(BasePlugin):
+    """Tasks plugin for managing activities (calls, notes, visits) linked to contacts."""
+
+    manifest = PluginManifest(
+        name="tasks",
+        version="1.0.0",
+        display_name="Tasks",
+        description="Manage free tasks/activities with status, priority, due dates, and contact links.",
+        dependencies=["permissions"],
+        routes=[
+            PluginRouteDef(
+                path="/api/v1/tasks",
+                module="app.plugins.builtins.tasks.routes",
+                router_attr="router",
+            ),
+        ],
+        events=[],
+        migrations=["0001_initial.sql"],
+        permissions=[
+            "tasks:read",
+            "tasks:write",
+            "tasks:delete",
+        ],
+        is_core=True,
+        menu_items=[
+            FrontendMenuItem(
+                label_key="nav.tasks",
+                label="Tasks",
+                path="/tasks",
+                icon="CheckSquare",
+                order=30,
+            ),
+        ],
+        page_routes=[
+            FrontendPageRoute(
+                path="/tasks",
+                component="@/pages/Tasks",
+                protected=True,
+                order=30,
+            ),
+        ],
+        cron_jobs=[
+            CronJobContribution(
+                name="tasks_due_reminder",
+                cron_expression="0 8 * * *",
+                job_type="custom",
+                target_name="tasks_due_reminder",
+                plugin_name="tasks",
+            ),
+        ],
+    )