feat: recurring tasks - auto-spawn next instance on completion

- Added recurrence field (daily/weekly/biweekly/monthly) to tasks schema
- Backend: auto-creates next task instance when recurring task completed
  - Copies title, description, assignee, project, tags, subtasks (unchecked)
  - Computes next due date based on frequency
  - Optional autoActivate to immediately activate next instance
- Frontend: recurrence picker in CreateTaskModal and TaskDetailPanel
- Recurrence badges (🔄) on TaskCard, KanbanBoard, TaskPage, DashboardPage
- Schema uses JSONB column (no migration needed, db:push on deploy)

frontend/src/lib/api.ts

@@ -1,4 +1,4 @@
-import type { Task, Project, ProjectWithTasks, VelocityStats } from "./types";
+import type { Task, Project, ProjectWithTasks, VelocityStats, Recurrence } from "./types";
 
 const BASE = "/api/tasks";
 
@@ -38,7 +38,7 @@ export async function reorderTasks(ids: string[], token?: string): Promise<void>
 }
 
 export async function createTask(
-  task: { title: string; description?: string; source?: string; priority?: string; status?: string; projectId?: string; dueDate?: string; estimatedHours?: number },
+  task: { title: string; description?: string; source?: string; priority?: string; status?: string; projectId?: string; dueDate?: string; estimatedHours?: number; recurrence?: Recurrence | null },
   token?: string
 ): Promise<Task> {
   const headers: Record<string, string> = { "Content-Type": "application/json" };