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/schemas.py

@@ -0,0 +1,63 @@
+"""Pydantic schemas for the Tasks plugin."""
+
+from __future__ import annotations
+
+from datetime import datetime
+from typing import Any
+
+from pydantic import BaseModel, Field
+
+
+class TaskCreate(BaseModel):
+    """Schema for creating a task."""
+    title: str = Field(..., min_length=1, max_length=255)
+    description: str | None = None
+    status: str = Field(default="open", pattern="^(open|in_progress|done)$")
+    priority: str = Field(default="medium", pattern="^(low|medium|high|urgent)$")
+    due_date: datetime | None = None
+    assigned_to: str | None = None
+    contact_id: str | None = None
+
+
+class TaskUpdate(BaseModel):
+    """Schema for updating a task."""
+    title: str | None = Field(default=None, min_length=1, max_length=255)
+    description: str | None = None
+    status: str | None = Field(default=None, pattern="^(open|in_progress|done)$")
+    priority: str | None = Field(default=None, pattern="^(low|medium|high|urgent)$")
+    due_date: datetime | None = None
+    assigned_to: str | None = None
+    contact_id: str | None = None
+
+
+class TaskAssignRequest(BaseModel):
+    """Schema for assigning a task."""
+    assigned_to: str = Field(..., description="User ID to assign the task to")
+
+
+class TaskStatusRequest(BaseModel):
+    """Schema for updating task status."""
+    status: str = Field(..., pattern="^(open|in_progress|done)$")
+
+
+class TaskResponse(BaseModel):
+    """Schema for task response."""
+    id: str
+    title: str
+    description: str | None = None
+    status: str
+    priority: str
+    due_date: datetime | None = None
+    assigned_to: str | None = None
+    contact_id: str | None = None
+    created_by: str | None = None
+    created_at: datetime
+    updated_at: datetime
+
+
+class TaskListResponse(BaseModel):
+    """Paginated task list response."""
+    items: list[TaskResponse]
+    total: int
+    page: int
+    page_size: int