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/handlers/auth_handler.go

@@ -406,5 +406,14 @@ func (h *AuthHandler) AppleSignIn(c *gin.Context) {
 		return
 	}
 
+	// Send welcome email for new users (async)
+	if response.IsNewUser && h.emailService != nil && response.User.Email != "" {
+		go func() {
+			if err := h.emailService.SendAppleWelcomeEmail(response.User.Email, response.User.FirstName); err != nil {
+				log.Error().Err(err).Str("email", response.User.Email).Msg("Failed to send Apple welcome email")
+			}
+		}()
+	}
+
 	c.JSON(http.StatusOK, response)
 }