fix(antigravity): fast-fail on proxy unavailable, temp-unschedule account

## Problem

When a proxy is unreachable, token refresh retries up to 4 times with
30s timeout each, causing requests to hang for ~2 minutes before
failing with a generic 502 error. The failed account is not marked,
so subsequent requests keep hitting it.

## Changes

### Proxy connection fast-fail
- Set TCP dial timeout to 5s and TLS handshake timeout to 5s on
  antigravity client, so proxy connectivity issues fail within 5s
  instead of 30s
- Reduce overall HTTP client timeout from 30s to 10s
- Export `IsConnectionError` for service-layer use
- Detect proxy connection errors in `RefreshToken` and return
  immediately with "proxy unavailable" error (no retries)

### Token refresh temp-unschedulable
- Add 8s context timeout for token refresh on request path
- Mark account as temp-unschedulable for 10min when refresh fails
  (both background `TokenRefreshService` and request-path
  `GetAccessToken`)
- Sync temp-unschedulable state to Redis cache for immediate
  scheduler effect
- Inject `TempUnschedCache` into `AntigravityTokenProvider`

### Account failover
- Return `UpstreamFailoverError` on `GetAccessToken` failure in
  `Forward`/`ForwardGemini` to trigger handler-level account switch
  instead of returning 502 directly

### Proxy probe alignment
- Apply same 5s dial/TLS timeout to shared `httpclient` pool
- Reduce proxy probe timeout from 30s to 10s

backend/internal/service/token_refresh_service.go

@@ -12,6 +12,9 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/config"
 )
 
+// tokenRefreshTempUnschedDuration token 刷新重试耗尽后临时不可调度的持续时间
+const tokenRefreshTempUnschedDuration = 10 * time.Minute
+
 // TokenRefreshService OAuth token自动刷新服务
 // 定期检查并刷新即将过期的token
 type TokenRefreshService struct {
@@ -317,7 +320,7 @@ func (s *TokenRefreshService) refreshWithRetry(ctx context.Context, account *Acc
 		}
 	}
 
-	// 可重试错误耗尽：仅记录日志，不标记 error（可能是临时网络问题，下个周期继续重试）
+	// 可重试错误耗尽：临时标记账号不可调度，避免请求路径反复命中已知失败的账号
 	slog.Warn("token_refresh.retry_exhausted",
 		"account_id", account.ID,
 		"platform", account.Platform,
@@ -325,6 +328,21 @@ func (s *TokenRefreshService) refreshWithRetry(ctx context.Context, account *Acc
 		"error", lastErr,
 	)
 
+	// 设置临时不可调度 10 分钟（不标记 error，保持 status=active 让下个刷新周期能继续尝试）
+	until := time.Now().Add(tokenRefreshTempUnschedDuration)
+	reason := fmt.Sprintf("token refresh retry exhausted: %v", lastErr)
+	if setErr := s.accountRepo.SetTempUnschedulable(ctx, account.ID, until, reason); setErr != nil {
+		slog.Warn("token_refresh.set_temp_unschedulable_failed",
+			"account_id", account.ID,
+			"error", setErr,
+		)
+	} else {
+		slog.Info("token_refresh.temp_unschedulable_set",
+			"account_id", account.ID,
+			"until", until.Format(time.RFC3339),
+		)
+	}
+
 	return lastErr
 }