refactor(service): 统一时间戳解析，支持多种格式

新增 Account.GetCredentialAsTime 方法，统一处理凭证中的时间戳字段，
兼容 RFC3339 字符串、Unix 时间戳字符串和数字类型。

- 重构 Claude/Gemini/Antigravity TokenRefresher.NeedsRefresh
- 移除重复的 parseExpiresAt/parseAntigravityExpiresAt 函数
- 简化 GetOpenAITokenExpiresAt 实现
- 新增 RFC3339 格式单元测试用例

backend/internal/service/token_refresher_test.go

@@ -33,6 +33,13 @@ func TestClaudeTokenRefresher_NeedsRefresh(t *testing.T) {
 			},
 			wantRefresh: true,
 		},
+		{
+			name: "expires_at as RFC3339 - expired",
+			credentials: map[string]any{
+				"expires_at": "1970-01-01T00:00:00Z", // RFC3339 格式，已过期
+			},
+			wantRefresh: true,
+		},
 		{
 			name: "expires_at as string - far future",
 			credentials: map[string]any{
@@ -47,6 +54,13 @@ func TestClaudeTokenRefresher_NeedsRefresh(t *testing.T) {
 			},
 			wantRefresh: false,
 		},
+		{
+			name: "expires_at as RFC3339 - far future",
+			credentials: map[string]any{
+				"expires_at": "2099-12-31T23:59:59Z", // RFC3339 格式，远未来
+			},
+			wantRefresh: false,
+		},
 		{
 			name:        "expires_at missing",
 			credentials: map[string]any{},