Add Apple Sign In welcome email, notification preferences on registration, and contractor sharing tests

- Send welcome email to new users who sign up via Apple Sign In
- Create notification preferences (all enabled) when new accounts are created
- Add comprehensive integration tests for contractor sharing:
  - Personal contractors only visible to creator
  - Residence-tied contractors visible to all users with residence access
  - Update/delete access control for shared contractors

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

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

internal/router/router.go

@@ -68,6 +68,7 @@ func SetupRouter(deps *Dependencies) *gin.Engine {
 
 	// Initialize services
 	authService := services.NewAuthService(userRepo, cfg)
+	authService.SetNotificationRepository(notificationRepo) // For creating notification preferences on registration
 	userService := services.NewUserService(userRepo)
 	residenceService := services.NewResidenceService(residenceRepo, userRepo, cfg)
 	residenceService.SetTaskRepository(taskRepo) // Wire up task repo for statistics