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/handlers/task_handler.go

@@ -295,6 +295,31 @@ func (h *TaskHandler) UnarchiveTask(c *gin.Context) {
 	c.JSON(http.StatusOK, response)
 }
 
+// QuickComplete handles POST /api/tasks/:id/quick-complete/
+// Lightweight endpoint for widget - just returns 200 OK on success
+func (h *TaskHandler) QuickComplete(c *gin.Context) {
+	user := c.MustGet(middleware.AuthUserKey).(*models.User)
+	taskID, err := strconv.ParseUint(c.Param("id"), 10, 32)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.invalid_task_id")})
+		return
+	}
+
+	err = h.taskService.QuickComplete(uint(taskID), user.ID)
+	if err != nil {
+		switch {
+		case errors.Is(err, services.ErrTaskNotFound):
+			c.JSON(http.StatusNotFound, gin.H{"error": i18n.LocalizedMessage(c, "error.task_not_found")})
+		case errors.Is(err, services.ErrTaskAccessDenied):
+			c.JSON(http.StatusForbidden, gin.H{"error": i18n.LocalizedMessage(c, "error.task_access_denied")})
+		default:
+			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		}
+		return
+	}
+	c.Status(http.StatusOK)
+}
+
 // === Task Completions ===
 
 // GetTaskCompletions handles GET /api/tasks/:id/completions/