Add Sign in with Apple authentication

- Add AppleSocialAuth model to store Apple ID linkages
- Create AppleAuthService for JWT verification with Apple's public keys
- Add AppleSignIn handler and route (POST /auth/apple-sign-in/)
- Implement account linking (links Apple ID to existing accounts by email)
- Add Redis caching for Apple public keys (24-hour TTL)
- Support private relay emails (@privaterelay.appleid.com)

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

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

internal/database/database.go

@@ -123,6 +123,7 @@ func Migrate() error {
 		&models.UserProfile{},
 		&models.ConfirmationCode{},
 		&models.PasswordResetCode{},
+		&models.AppleSocialAuth{},
 
 		// Admin users (separate from app users)
 		&models.AdminUser{},