feat(gateway): 可观测性 —— Prometheus 指标 + 结构化日志 + 探针

往"生产可运维"推一步（网关前门）：
- Prometheus /metrics：sundynix_http_requests_total{method,route,status}、
  request_duration_seconds 直方图、requests_in_flight。route 用 c.FullPath()
  路由模板（/tasks/:id/...）避免按真实路径高基数。
- 结构化访问日志：slog JSON 到 stderr（request_id/method/route/status/latency_ms/
  ip/uid/bytes），替代 gin 默认文本日志；gin.New()+Recovery 自管中间件链。
- RequestID 中间件：生成/透传 X-Request-ID，写上下文+响应头，供日志关联。
- 探针：/healthz（liveness，不查依赖）、/readyz（readiness，DB+Redis 就绪才 200，
  否则 503），供 k8s 等导流判断；/api/v1/health 深度聚合保留。
- 三个根端点不挂业务鉴权（/metrics 生产应由网络层限制抓取来源）。

验证：单测（计数 +1 / X-Request-ID 生成与透传）；实跑 /healthz 200、/readyz 200
（db,redis ready）、/metrics 输出真实指标、访问日志 JSON 正常、X-Request-ID 回写。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sundynix-gateway/go.mod

@@ -5,9 +5,12 @@ go 1.25.0
 require (
 	github.com/bwmarrin/snowflake v0.3.0
 	github.com/gin-gonic/gin v1.10.0
+	github.com/golang-jwt/jwt/v5 v5.3.1
 	github.com/minio/minio-go/v7 v7.2.0
+	github.com/prometheus/client_golang v1.23.2
 	github.com/redis/go-redis/v9 v9.20.0
 	github.com/sundynix/sundynix-shared v0.0.0
+	golang.org/x/crypto v0.53.0
 	gorm.io/driver/postgres v1.6.0
 	gorm.io/gorm v1.31.1
 )
@@ -15,6 +18,7 @@ require (
 replace github.com/sundynix/sundynix-shared => ../sundynix-shared
 
 require (
+	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/bytedance/gopkg v0.1.3 // indirect
 	github.com/bytedance/sonic v1.15.0 // indirect
 	github.com/bytedance/sonic/loader v0.5.0 // indirect
@@ -27,7 +31,6 @@ require (
 	github.com/go-playground/universal-translator v0.18.1 // indirect
 	github.com/go-playground/validator/v10 v10.20.0 // indirect
 	github.com/goccy/go-json v0.10.2 // indirect
-	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
@@ -40,26 +43,31 @@ require (
 	github.com/klauspost/cpuid/v2 v2.2.11 // indirect
 	github.com/klauspost/crc32 v1.3.0 // indirect
 	github.com/kr/text v0.2.0 // indirect
+	github.com/kylelemons/godebug v1.1.0 // indirect
 	github.com/leodido/go-urn v1.4.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/minio/crc64nvme v1.1.1 // indirect
 	github.com/minio/md5-simd v1.1.2 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect
+	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/nats-io/nats.go v1.37.0 // indirect
 	github.com/nats-io/nkeys v0.4.7 // indirect
 	github.com/nats-io/nuid v1.0.1 // indirect
 	github.com/pelletier/go-toml/v2 v2.3.1 // indirect
 	github.com/philhofer/fwd v1.2.0 // indirect
+	github.com/prometheus/client_model v0.6.2 // indirect
+	github.com/prometheus/common v0.66.1 // indirect
+	github.com/prometheus/procfs v0.16.1 // indirect
 	github.com/rs/xid v1.6.0 // indirect
 	github.com/tinylib/msgp v1.6.1 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.2.12 // indirect
 	github.com/zeebo/xxh3 v1.1.0 // indirect
 	go.uber.org/atomic v1.11.0 // indirect
+	go.yaml.in/yaml/v2 v2.4.2 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/arch v0.11.0 // indirect
-	golang.org/x/crypto v0.53.0 // indirect
 	golang.org/x/net v0.55.0 // indirect
 	golang.org/x/sync v0.21.0 // indirect
 	golang.org/x/sys v0.46.0 // indirect