feat(server): API relay gateway backend M0-M4

Gin + GORM + pure-Go SQLite. Users/auth (JWT), API key management with
quotas, proxy gateway with weighted channel failover and health checks,
usage/billing ledger, cross-protocol conversion (Anthropic Messages /
OpenAI Chat Completions / OpenAI Responses), and channel/model admin API.
Channels declare native API formats and auto-convert the rest.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/internal/channel/service.go

@@ -0,0 +1,156 @@
+package channel
+
+import (
+	"errors"
+	"math/rand"
+	"sort"
+	"sync"
+
+	"go.uber.org/zap"
+	"gorm.io/gorm"
+
+	"openteam/server/internal/config"
+	"openteam/server/internal/pkg/crypto"
+	"openteam/server/internal/store"
+)
+
+var ErrNoChannel = errors.New("no available channel for model")
+
+// Service manages channels, model bindings and load-balanced selection.
+type Service struct {
+	db  *gorm.DB
+	cfg *config.Config
+	log *zap.Logger
+
+	mu     sync.RWMutex
+	sem    map[int64]chan struct{} // per-channel concurrency limiter
+	master string
+}
+
+func NewService(db *gorm.DB, cfg *config.Config, log *zap.Logger) *Service {
+	return &Service{
+		db: db, cfg: cfg, log: log,
+		sem:    map[int64]chan struct{}{},
+		master: cfg.MasterKey,
+	}
+}
+
+// DecryptKey decrypts a channel's stored upstream API key.
+func (s *Service) DecryptKey(enc string) (string, error) {
+	return crypto.Decrypt(enc, s.master)
+}
+
+// Acquire takes a concurrency slot for a channel (blocks if saturated).
+func (s *Service) Acquire(channelID int64) (func(), error) {
+	s.mu.Lock()
+	lim, ok := s.sem[channelID]
+	if !ok {
+		var rec store.Channel
+		if err := s.db.First(&rec, channelID).Error; err != nil {
+			s.mu.Unlock()
+			return nil, err
+		}
+		limit := rec.MaxConcurrency
+		if limit <= 0 {
+			limit = 100
+		}
+		lim = make(chan struct{}, limit)
+		s.sem[channelID] = lim
+	}
+	s.mu.Unlock()
+	lim <- struct{}{}
+	return func() { <-lim }, nil
+}
+
+// ResolveModel returns the model registry record.
+func (s *Service) ResolveModel(name string) (*store.Model, error) {
+	var m store.Model
+	if err := s.db.Where("name = ? AND enabled = ?", name, true).First(&m).Error; err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return nil, errors.New("model not found: " + name)
+		}
+		return nil, err
+	}
+	return &m, nil
+}
+
+// SelectChannel picks a healthy channel bound to the given model.
+// Channels with higher priority (lower number) and higher weight win;
+// cooldown channels are skipped, degraded channels are deprioritized.
+// Channels in exclude (already tried in this request) are skipped.
+func (s *Service) SelectChannel(modelID int64, exclude map[int64]bool) (*store.Channel, *store.ChannelModelBinding, error) {
+	var bindings []store.ChannelModelBinding
+	if err := s.db.Preload("Channel").Where("model_id = ?", modelID).Find(&bindings).Error; err != nil {
+		return nil, nil, err
+	}
+	var candidates []struct {
+		ch *store.Channel
+		b  *store.ChannelModelBinding
+		score float64
+	}
+	for i := range bindings {
+		ch := &bindings[i].Channel
+		if !ch.Enabled {
+			continue
+		}
+		if ch.HealthStatus == "cooldown" {
+			continue
+		}
+		if exclude[ch.ID] {
+			continue
+		}
+		score := float64(bindings[i].Weight)
+		if ch.HealthStatus == "degraded" {
+			score *= 0.1
+		}
+		candidates = append(candidates, struct {
+			ch *store.Channel
+			b  *store.ChannelModelBinding
+			score float64
+		}{ch: ch, b: &bindings[i], score: score})
+	}
+	if len(candidates) == 0 {
+		return nil, nil, ErrNoChannel
+	}
+	// Sort by priority asc, then weight desc.
+	sort.SliceStable(candidates, func(i, j int) bool {
+		if candidates[i].ch.Priority != candidates[j].ch.Priority {
+			return candidates[i].ch.Priority < candidates[j].ch.Priority
+		}
+		return candidates[i].score > candidates[j].score
+	})
+	// Weighted random pick among the top priority group.
+	total := 0.0
+	for _, c := range candidates {
+		total += c.score
+	}
+	if total <= 0 {
+		return candidates[0].ch, candidates[0].b, nil
+	}
+	pick := rand.Float64() * total
+	for _, c := range candidates {
+		pick -= c.score
+		if pick <= 0 {
+			return c.ch, c.b, nil
+		}
+	}
+	return candidates[0].ch, candidates[0].b, nil
+}
+
+// ListModelsForChannel returns the model names a channel serves.
+func (s *Service) ListModelsForChannel(channelID int64) ([]string, error) {
+	var names []string
+	err := s.db.Table("channel_model_bindings").
+		Joins("JOIN models ON models.id = channel_model_bindings.model_id").
+		Where("channel_model_bindings.channel_id = ? AND models.enabled = ?", channelID, true).
+		Order("models.sort ASC, models.id ASC").
+		Pluck("models.name", &names).Error
+	return names, err
+}
+
+// ListEnabledModels returns all enabled models for GET /v1/models.
+func (s *Service) ListEnabledModels() ([]store.Model, error) {
+	var models []store.Model
+	err := s.db.Where("enabled = ?", true).Order("sort ASC, id ASC").Find(&models).Error
+	return models, err
+}