Add custom interval days support for task frequency

Backend changes:
- Add "Custom" frequency option to database seeds
- Add custom_interval_days field to Task model
- Update task DTOs to accept custom_interval_days
- Update task service to use custom_interval_days for next due date calculation

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/models/task.go

@@ -67,6 +67,7 @@ type Task struct {
 	Priority      *TaskPriority    `gorm:"foreignKey:PriorityID" json:"priority,omitempty"`
 	FrequencyID   *uint            `gorm:"column:frequency_id;index" json:"frequency_id"`
 	Frequency     *TaskFrequency   `gorm:"foreignKey:FrequencyID" json:"frequency,omitempty"`
+	CustomIntervalDays *int        `gorm:"column:custom_interval_days" json:"custom_interval_days"` // For "Custom" frequency, user-specified days
 
 	// In Progress flag - replaces status lookup
 	InProgress    bool             `gorm:"column:in_progress;default:false;index" json:"in_progress"`