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/dto/responses/auth.go

@@ -155,3 +155,19 @@ func NewRegisterResponse(token string, user *models.User) RegisterResponse {
 		Message: "Registration successful. Please check your email to verify your account.",
 	}
 }
+
+// AppleSignInResponse represents the Apple Sign In response
+type AppleSignInResponse struct {
+	Token     string       `json:"token"`
+	User      UserResponse `json:"user"`
+	IsNewUser bool         `json:"is_new_user"`
+}
+
+// NewAppleSignInResponse creates an AppleSignInResponse
+func NewAppleSignInResponse(token string, user *models.User, isNewUser bool) AppleSignInResponse {
+	return AppleSignInResponse{
+		Token:     token,
+		User:      NewUserResponse(user),
+		IsNewUser: isNewUser,
+	}
+}