Add IsFree subscription toggle to bypass all tier limitations

- Add IsFree boolean field to UserSubscription model
- When IsFree is true, user sees limitations_enabled=false regardless of global setting
- CheckLimit() bypasses all limit checks for IsFree users
- Add admin endpoint GET /api/admin/subscriptions/user/:user_id
- Add IsFree toggle to admin user detail page under Subscription card
- Add database migration 004_subscription_is_free
- Add integration tests for IsFree functionality
- Add task kanban categorization documentation

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

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

internal/models/subscription.go

@@ -42,6 +42,9 @@ type UserSubscription struct {
 	// Tracking
 	CancelledAt       *time.Time       `gorm:"column:cancelled_at" json:"cancelled_at"`
 	Platform          string           `gorm:"column:platform;size:10" json:"platform"` // ios, android
+
+	// Admin override - bypasses all limitations regardless of global settings
+	IsFree            bool             `gorm:"column:is_free;default:false" json:"is_free"`
 }
 
 // TableName returns the table name for GORM