fix: SSAFYTokenResponse.expires_in 타입을 int로 수정

SSAFY 서버가 expires_in을 숫자로 반환하므로 string에서 int로 변경.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/auth/model.go

@@ -29,7 +29,7 @@ type SSAFYTokenResponse struct {
 	TokenType             string `json:"token_type"`
 	AccessToken           string `json:"access_token"`
 	Scope                 string `json:"scope"`
-	ExpiresIn             string `json:"expires_in"`
+	ExpiresIn             int    `json:"expires_in"`
 	RefreshToken          string `json:"refresh_token"`
 	RefreshTokenExpiresIn int    `json:"refresh_token_expires_in"`
 }