chore(logging): 完成后端日志审计与结构化迁移

- 将高密度服务与处理器日志迁移到新日志系统（LegacyPrintf/结构化日志）
- 增加 stdlog bridge 与兼容测试，保留旧日志捕获能力
- 将 OpenAI 断流告警改为结构化 Warn 并改造对应测试为 sink 捕获
- 补齐后端相关文件 logger 引用并通过全量 go test

backend/internal/handler/gemini_v1beta_handler.go

@@ -8,7 +8,6 @@ import (
 	"encoding/json"
 	"errors"
 	"io"
-	"log"
 	"net/http"
 	"regexp"
 	"strings"
@@ -20,11 +19,13 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/pkg/gemini"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/googleapi"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/ip"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/logger"
 	"github.com/Wei-Shaw/sub2api/internal/server/middleware"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 	"github.com/google/uuid"
 
 	"github.com/gin-gonic/gin"
+	"go.uber.org/zap"
 )
 
 // geminiCLITmpDirRegex 用于从 Gemini CLI 请求体中提取 tmp 目录的哈希值
@@ -143,6 +144,13 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 		googleError(c, http.StatusInternalServerError, "User context not found")
 		return
 	}
+	reqLog := requestLogger(
+		c,
+		"handler.gemini_v1beta.models",
+		zap.Int64("user_id", authSubject.UserID),
+		zap.Int64("api_key_id", apiKey.ID),
+		zap.Any("group_id", apiKey.GroupID),
+	)
 
 	// 检查平台：优先使用强制平台（/antigravity 路由，中间件已设置 request.Context），否则要求 gemini 分组
 	if !middleware.HasForcePlatform(c) {
@@ -159,6 +167,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	}
 
 	stream := action == "streamGenerateContent"
+	reqLog = reqLog.With(zap.String("model", modelName), zap.String("action", action), zap.Bool("stream", stream))
 
 	body, err := io.ReadAll(c.Request.Body)
 	if err != nil {
@@ -187,8 +196,9 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	canWait, err := geminiConcurrency.IncrementWaitCount(c.Request.Context(), authSubject.UserID, maxWait)
 	waitCounted := false
 	if err != nil {
-		log.Printf("Increment wait count failed: %v", err)
+		reqLog.Warn("gemini.user_wait_counter_increment_failed", zap.Error(err))
 	} else if !canWait {
+		reqLog.Info("gemini.user_wait_queue_full", zap.Int("max_wait", maxWait))
 		googleError(c, http.StatusTooManyRequests, "Too many pending requests, please retry later")
 		return
 	}
@@ -208,6 +218,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	}
 	userReleaseFunc, err := geminiConcurrency.AcquireUserSlotWithWait(c, authSubject.UserID, authSubject.Concurrency, stream, &streamStarted)
 	if err != nil {
+		reqLog.Warn("gemini.user_slot_acquire_failed", zap.Error(err))
 		googleError(c, http.StatusTooManyRequests, err.Error())
 		return
 	}
@@ -223,6 +234,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 
 	// 2) billing eligibility check (after wait)
 	if err := h.billingCacheService.CheckBillingEligibility(c.Request.Context(), apiKey.User, apiKey, apiKey.Group, subscription); err != nil {
+		reqLog.Info("gemini.billing_eligibility_check_failed", zap.Error(err))
 		status, _, message := billingErrorDetails(err)
 		googleError(c, status, message)
 		return
@@ -296,8 +308,11 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 					matchedDigestChain = foundMatchedChain
 					sessionBoundAccountID = foundAccountID
 					geminiSessionUUID = foundUUID
-					log.Printf("[Gemini] Digest fallback matched: uuid=%s, accountID=%d, chain=%s",
-						safeShortPrefix(foundUUID, 8), foundAccountID, truncateDigestChain(geminiDigestChain))
+					reqLog.Info("gemini.digest_fallback_matched",
+						zap.String("session_uuid_prefix", safeShortPrefix(foundUUID, 8)),
+						zap.Int64("account_id", foundAccountID),
+						zap.String("digest_chain", truncateDigestChain(geminiDigestChain)),
+					)
 
 					// 关键：如果原 sessionKey 为空，使用 prefixHash + uuid 作为 sessionKey
 					// 这样 SelectAccountWithLoadAwareness 的粘性会话逻辑会优先使用匹配到的账号
@@ -346,7 +361,10 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 			// 谷歌上游 503 (MODEL_CAPACITY_EXHAUSTED) 通常是暂时性的，等几秒就能恢复。
 			if lastFailoverErr != nil && lastFailoverErr.StatusCode == http.StatusServiceUnavailable && switchCount <= maxAccountSwitches {
 				if sleepAntigravitySingleAccountBackoff(c.Request.Context(), switchCount) {
-					log.Printf("Antigravity single-account 503 retry: clearing failed accounts, retry %d/%d", switchCount, maxAccountSwitches)
+					reqLog.Warn("gemini.single_account_retrying",
+						zap.Int("retry_count", switchCount),
+						zap.Int("max_retries", maxAccountSwitches),
+					)
 					failedAccountIDs = make(map[int64]struct{})
 					// 设置 context 标记，让 Service 层预检查等待限流过期而非直接切换
 					ctx := context.WithValue(c.Request.Context(), ctxkey.SingleAccountRetry, true)
@@ -363,13 +381,19 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 		// 检测账号切换：如果粘性会话绑定的账号与当前选择的账号不同，清除 thoughtSignature
 		// 注意：Gemini 原生 API 的 thoughtSignature 与具体上游账号强相关；跨账号透传会导致 400。
 		if sessionBoundAccountID > 0 && sessionBoundAccountID != account.ID {
-			log.Printf("[Gemini] Sticky session account switched: %d -> %d, cleaning thoughtSignature", sessionBoundAccountID, account.ID)
+			reqLog.Info("gemini.sticky_session_account_switched",
+				zap.Int64("from_account_id", sessionBoundAccountID),
+				zap.Int64("to_account_id", account.ID),
+				zap.Bool("clean_thought_signature", true),
+			)
 			body = service.CleanGeminiNativeThoughtSignatures(body)
 			sessionBoundAccountID = account.ID
 		} else if sessionKey != "" && sessionBoundAccountID == 0 && !cleanedForUnknownBinding && bytes.Contains(body, []byte(`"thoughtSignature"`)) {
 			// 无缓存绑定但请求里已有 thoughtSignature：常见于缓存丢失/TTL 过期后，客户端继续携带旧签名。
 			// 为避免第一次转发就 400，这里做一次确定性清理，让新账号重新生成签名链路。
-			log.Printf("[Gemini] Sticky session binding missing, cleaning thoughtSignature proactively")
+			reqLog.Info("gemini.sticky_session_binding_missing",
+				zap.Bool("clean_thought_signature", true),
+			)
 			body = service.CleanGeminiNativeThoughtSignatures(body)
 			cleanedForUnknownBinding = true
 			sessionBoundAccountID = account.ID
@@ -388,9 +412,12 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 			accountWaitCounted := false
 			canWait, err := geminiConcurrency.IncrementAccountWaitCount(c.Request.Context(), account.ID, selection.WaitPlan.MaxWaiting)
 			if err != nil {
-				log.Printf("Increment account wait count failed: %v", err)
+				reqLog.Warn("gemini.account_wait_counter_increment_failed", zap.Int64("account_id", account.ID), zap.Error(err))
 			} else if !canWait {
-				log.Printf("Account wait queue full: account=%d", account.ID)
+				reqLog.Info("gemini.account_wait_queue_full",
+					zap.Int64("account_id", account.ID),
+					zap.Int("max_waiting", selection.WaitPlan.MaxWaiting),
+				)
 				googleError(c, http.StatusTooManyRequests, "Too many pending requests, please retry later")
 				return
 			}
@@ -412,6 +439,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				&streamStarted,
 			)
 			if err != nil {
+				reqLog.Warn("gemini.account_slot_acquire_failed", zap.Int64("account_id", account.ID), zap.Error(err))
 				googleError(c, http.StatusTooManyRequests, err.Error())
 				return
 			}
@@ -420,7 +448,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				accountWaitCounted = false
 			}
 			if err := h.gatewayService.BindStickySession(c.Request.Context(), apiKey.GroupID, sessionKey, account.ID); err != nil {
-				log.Printf("Bind sticky session failed: %v", err)
+				reqLog.Warn("gemini.bind_sticky_session_failed", zap.Int64("account_id", account.ID), zap.Error(err))
 			}
 		}
 		// 账号槽位/等待计数需要在超时或断开时安全回收
@@ -454,7 +482,12 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				}
 				lastFailoverErr = failoverErr
 				switchCount++
-				log.Printf("Gemini account %d: upstream error %d, switching account %d/%d", account.ID, failoverErr.StatusCode, switchCount, maxAccountSwitches)
+				reqLog.Warn("gemini.upstream_failover_switching",
+					zap.Int64("account_id", account.ID),
+					zap.Int("upstream_status", failoverErr.StatusCode),
+					zap.Int("switch_count", switchCount),
+					zap.Int("max_switches", maxAccountSwitches),
+				)
 				if account.Platform == service.PlatformAntigravity {
 					if !sleepFailoverDelay(c.Request.Context(), switchCount) {
 						return
@@ -463,7 +496,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				continue
 			}
 			// ForwardNative already wrote the response
-			log.Printf("Gemini native forward failed: %v", err)
+			reqLog.Error("gemini.forward_failed", zap.Int64("account_id", account.ID), zap.Error(err))
 			return
 		}
 
@@ -482,7 +515,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				account.ID,
 				matchedDigestChain,
 			); err != nil {
-				log.Printf("[Gemini] Failed to save digest session: %v", err)
+				reqLog.Warn("gemini.digest_session_save_failed", zap.Int64("account_id", account.ID), zap.Error(err))
 			}
 		}
 
@@ -504,9 +537,20 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				ForceCacheBilling:     fcb,
 				APIKeyService:         h.apiKeyService,
 			}); err != nil {
-				log.Printf("Record usage failed: %v", err)
+				logger.L().With(
+					zap.String("component", "handler.gemini_v1beta.models"),
+					zap.Int64("user_id", authSubject.UserID),
+					zap.Int64("api_key_id", apiKey.ID),
+					zap.Any("group_id", apiKey.GroupID),
+					zap.String("model", modelName),
+					zap.Int64("account_id", usedAccount.ID),
+				).Error("gemini.record_usage_failed", zap.Error(err))
 			}
 		}(result, account, userAgent, clientIP, forceCacheBilling)
+		reqLog.Debug("gemini.request_completed",
+			zap.Int64("account_id", account.ID),
+			zap.Int("switch_count", switchCount),
+		)
 		return
 	}
 }