Add quick-complete endpoint for iOS widget task completion

- Add lightweight POST /api/tasks/:id/quick-complete/ endpoint
- Creates task completion with minimal processing for widget use
- Returns only 200 OK on success (no response body)
- Updates task status and next_due_date based on frequency
- Sends completion notification asynchronously

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/router/router.go

@@ -298,6 +298,7 @@ func setupTaskRoutes(api *gin.RouterGroup, taskHandler *handlers.TaskHandler) {
 		tasks.POST("/:id/uncancel/", taskHandler.UncancelTask)
 		tasks.POST("/:id/archive/", taskHandler.ArchiveTask)
 		tasks.POST("/:id/unarchive/", taskHandler.UnarchiveTask)
+		tasks.POST("/:id/quick-complete/", taskHandler.QuickComplete)
 		tasks.GET("/:id/completions/", taskHandler.GetTaskCompletions)
 	}