M5: 渠道体系(负载均衡+并发控制+健康检查+故障转移)

- channel.Candidates 按模型绑定取候选 + Pick 加权随机负载均衡
- TryAcquire 每渠道并发信号量, 满载溢出到其他候选
- HealthMonitor 后台定时探测, 连续失败进 cooldown, 恢复放回(可配 interval/threshold)
- doProxy 遍历候选故障转移: 网络错误/429/5xx/超时且未写出响应头时安全重试;
  400 等业务错误透传, 流式写出首字节后放弃重试
- 单测覆盖候选过滤/加权/并发/健康状态机
- E2E: 杀上游自动切换、cooldown、恢复、并发溢出 10/10

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

server/internal/app/app.go

@@ -6,6 +6,7 @@ import (
 	"log"
 	"time"
 
+	"github.com/openteam/server/internal/channel"
 	"github.com/openteam/server/internal/config"
 	"github.com/openteam/server/internal/pkg/crypto"
 	"github.com/openteam/server/internal/pkg/jwt"
@@ -21,7 +22,10 @@ type App struct {
 	Enc       *crypto.Encryptor
 	JWT       *jwt.Manager
 	Usage     *usage.Recorder
+	Health    *channel.HealthMonitor
 	startedAt time.Time
+	ctx       context.Context
+	cancel    context.CancelFunc
 }
 
 func New(cfg *config.Config) (*App, error) {
@@ -38,11 +42,18 @@ func New(cfg *config.Config) (*App, error) {
 		JWT:       jwt.NewManager(cfg.JWT.Secret, cfg.JWT.Issuer, cfg.JWT.AccessTTL, cfg.JWT.RefreshTTL),
 		startedAt: time.Now(),
 	}
+	a.ctx, a.cancel = context.WithCancel(context.Background())
 	a.Usage = usage.NewRecorder(db)
 
 	if err := a.Seed(); err != nil {
 		return nil, err
 	}
+
+	a.Health = channel.NewHealthMonitor(db, a.Enc, channel.HealthConfig{
+		Interval:      cfg.Proxy.HealthInterval,
+		FailThreshold: cfg.Proxy.HealthFailThreshold,
+	})
+	a.Health.Start(a.ctx)
 	return a, nil
 }
 
@@ -116,6 +127,7 @@ func (a *App) Seed() error {
 }
 
 func (a *App) Shutdown(ctx context.Context) {
+	a.cancel()
 	a.Usage.Close()
 	if sqlDB, err := a.DB.DB(); err == nil {
 		_ = sqlDB.Close()