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/services/subscription_service.go

@@ -95,12 +95,19 @@ func (s *SubscriptionService) GetSubscriptionStatus(userID uint) (*SubscriptionS
 		return nil, err
 	}
 
+	// Determine if limitations are enabled for this user
+	// If user has IsFree flag, always return false (no limitations)
+	limitationsEnabled := settings.EnableLimitations
+	if sub.IsFree {
+		limitationsEnabled = false
+	}
+
 	// Build flattened response (KMM expects subscription fields at top level)
 	resp := &SubscriptionStatusResponse{
 		AutoRenew:          sub.AutoRenew,
 		Limits:             limitsMap,
 		Usage:              usage,
-		LimitationsEnabled: settings.EnableLimitations,
+		LimitationsEnabled: limitationsEnabled,
 	}
 
 	// Format dates if present
@@ -161,7 +168,7 @@ func (s *SubscriptionService) CheckLimit(userID uint, limitType string) error {
 		return err
 	}
 
-	// If limitations are disabled, allow everything
+	// If limitations are disabled globally, allow everything
 	if !settings.EnableLimitations {
 		return nil
 	}
@@ -171,6 +178,11 @@ func (s *SubscriptionService) CheckLimit(userID uint, limitType string) error {
 		return err
 	}
 
+	// IsFree users bypass all limitations
+	if sub.IsFree {
+		return nil
+	}
+
 	// Pro users have unlimited access
 	if sub.IsPro() {
 		return nil