feat(observability): 日志聚合后端 — logger环形缓冲 + GET /admin/observability/logs

- internal/logger/ring.go: Entry{time,level,source,msg} + RingBuffer(2000,满覆盖最旧,不阻塞)+ringHandler(slog旁路,Enabled委托内层,sourceOf提取source=再回落Scoped); Options.Ring非nil才包
- gateway/observability_logs.go: GET /admin/observability/logs?level=&limit= (writeAuth owner/admin) → {data:[{time(RFC3339 UTC),level,source,msg}]} 最新在前; level阈值过滤(warn=warn+error,已标注取舍); limit缺省200上限1000; 未知level400/未注入501
- routes.go注册 + observability.go SetLogBuffer(atomic.Pointer) + cmd/cyrene main.go/wireGatewayLogBuffer接线(logger先于gateway)
- 控制台/文件输出零变化; 记录只源自已产出日志,不额外打印敏感
- Docs/api-client v0.35; 测试(ring 10 + observability_logs 8 + cmd端到端2); build/vet/test绿(56)+win/darwin交叉编译

internal/services/gateway/observability.go

@@ -3,9 +3,11 @@ package gateway
 import (
 	"net/http"
 	"strconv"
+	"sync/atomic"
 	"time"
 
 	apperr "git.yeij.top/AskaEth/Cyrene_v2/internal/errors"
+	"git.yeij.top/AskaEth/Cyrene_v2/internal/logger"
 	"git.yeij.top/AskaEth/Cyrene_v2/internal/runtime/trace"
 )
 
@@ -17,12 +19,17 @@ const (
 )
 
 // observability bundles the gateway's live admin instrumentation: the request
-// metrics counter, the trace span ring buffer and the audit store. It is built in
-// New and is safe for concurrent use.
+// metrics counter, the trace span ring buffer, the audit store and the
+// process-wide log ring buffer. It is built in New and is safe for concurrent
+// use.
 type observability struct {
 	metrics  *MetricsCollector
 	traceBuf *RingBuffer
 	audit    AuditStore
+	// logBuf is the logger's log ring buffer behind the admin log-tail endpoint.
+	// The composition root injects it via SetLogBuffer once the gateway exists;
+	// until then the endpoint answers 501 rather than an empty-but-fake tail.
+	logBuf atomic.Pointer[logger.RingBuffer]
 }
 
 // newObservability builds the instrumentation, falling back to an in-memory audit