Add custom_interval_days to TaskResponse DTO

Field existed in Task model but was missing from API response.
Aligns Go API contract with KMM mobile model.

internal/dto/responses/task.go

@@ -85,6 +85,7 @@ type TaskResponse struct {
 	Priority        *TaskPriorityResponse    `json:"priority,omitempty"`
 	FrequencyID        *uint                    `json:"frequency_id"`
 	Frequency          *TaskFrequencyResponse   `json:"frequency,omitempty"`
+	CustomIntervalDays *int                     `json:"custom_interval_days"` // For "Custom" frequency, user-specified days
 	InProgress         bool                     `json:"in_progress"`
 	DueDate         *time.Time               `json:"due_date"`
 	NextDueDate     *time.Time               `json:"next_due_date"` // For recurring tasks, updated after each completion
@@ -237,6 +238,7 @@ func newTaskResponseInternal(t *models.Task, daysThreshold int, now time.Time) T
 		CategoryID:      t.CategoryID,
 		PriorityID:      t.PriorityID,
 		FrequencyID:        t.FrequencyID,
+		CustomIntervalDays: t.CustomIntervalDays,
 		InProgress:         t.InProgress,
 		AssignedToID:    t.AssignedToID,
 		DueDate:         t.DueDate,