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/pages/TaskPage.tsx

@@ -248,6 +248,11 @@ export function TaskPage() {
                     📁 {project.name}
                   </span>
                 )}
+                {task.recurrence && (
+                  <span className="text-xs text-teal-600 dark:text-teal-400 bg-teal-100 dark:bg-teal-900/30 px-2 py-0.5 rounded-full font-medium">
+                    🔄 {task.recurrence.frequency === "biweekly" ? "Bi-weekly" : task.recurrence.frequency.charAt(0).toUpperCase() + task.recurrence.frequency.slice(1)}
+                  </span>
+                )}
                 {task.tags?.map(tag => (
                   <span key={tag} className="text-xs text-violet-600 dark:text-violet-400 bg-violet-100 dark:bg-violet-900/30 px-2 py-0.5 rounded-full font-medium">
                     🏷️ {tag}
@@ -623,6 +628,14 @@ export function TaskPage() {
                     <span className="text-gray-700 dark:text-gray-300 font-medium">⏱ {task.estimatedHours}h</span>
                   </div>
                 )}
+                {task.recurrence && (
+                  <div className="flex justify-between">
+                    <span className="text-gray-500 dark:text-gray-400">Recurrence</span>
+                    <span className="text-teal-600 dark:text-teal-400 font-medium">
+                      🔄 {task.recurrence.frequency === "biweekly" ? "Bi-weekly" : task.recurrence.frequency.charAt(0).toUpperCase() + task.recurrence.frequency.slice(1)}
+                    </span>
+                  </div>
+                )}
                 {task.tags?.length > 0 && (
                   <div>
                     <span className="text-gray-500 dark:text-gray-400 block mb-1">Tags</span>