refactor: move backend files to backend/ directory

Reorganize project structure:
- backend/cmd/openteam/ — entry point
- backend/internal/ — core packages
- backend/middleware/ — HTTP middleware
- backend/router/ — route setup
- backend/wire/ — dependency injection
- backend/pkg/ — shared utilities
- backend/go.mod, go.sum — Go module files

Updated Makefile to work from backend/ directory.
Removed old lowercase makefile.

backend/internal/channel/channel.go

@@ -0,0 +1,221 @@
+package channel
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"math/rand"
+	"opencatd-open/internal/dao"
+	"opencatd-open/internal/store"
+	"opencatd-open/internal/pkg/crypto"
+	"sync"
+	"time"
+)
+
+type Service struct {
+	channelDAO *dao.ChannelDAO
+	modelDAO   *dao.ModelDAO
+
+	// Health tracking
+	mu           sync.RWMutex
+	healthStatus map[uint64]*channelHealth
+}
+
+type channelHealth struct {
+	status      string
+	consecutive int
+	lastCheck   time.Time
+	cooldown    time.Time
+}
+
+func NewService(channelDAO *dao.ChannelDAO, modelDAO *dao.ModelDAO) *Service {
+	return &Service{
+		channelDAO:  channelDAO,
+		modelDAO:    modelDAO,
+		healthStatus: make(map[uint64]*channelHealth),
+	}
+}
+
+// SelectChannel selects the best channel for a given model using weighted random selection
+func (s *Service) SelectChannel(ctx context.Context, modelName string) (*store.Channel, error) {
+	channels, err := s.channelDAO.GetEnabledChannelsByModel(modelName)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get channels for model %s: %w", modelName, err)
+	}
+	if len(channels) == 0 {
+		return nil, fmt.Errorf("no enabled channels for model: %s", modelName)
+	}
+
+	// Filter out unhealthy channels
+	candidates := s.filterHealthy(channels)
+	if len(candidates) == 0 {
+		// If all channels are unhealthy, try the first one anyway
+		candidates = channels[:1]
+	}
+
+	// Weighted random selection
+	totalWeight := 0
+	for _, ch := range candidates {
+		totalWeight += ch.Weight
+	}
+	if totalWeight == 0 {
+		return candidates[0], nil
+	}
+
+	r := rand.Intn(totalWeight)
+	for _, ch := range candidates {
+		r -= ch.Weight
+		if r < 0 {
+			return ch, nil
+		}
+	}
+
+	return candidates[0], nil
+}
+
+// GetChannelByKeyID decrypts the API key for a channel
+func (s *Service) GetChannelByKeyID(ctx context.Context, channelID uint64) (*store.Channel, error) {
+	ch, err := s.channelDAO.GetByID(channelID)
+	if err != nil {
+		return nil, err
+	}
+	return ch, nil
+}
+
+// GetAPIKey decrypts the channel's API key
+func (s *Service) GetAPIKey(ch *store.Channel) (string, error) {
+	return crypto.Decrypt(ch.APIKeyEnc)
+}
+
+// RecordSuccess records a successful request to a channel
+func (s *Service) RecordSuccess(channelID uint64) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	h := s.getOrCreateHealth(channelID)
+	h.consecutive = 0
+	h.status = store.ChannelHealthHealthy
+	h.lastCheck = time.Now()
+}
+
+// RecordFailure records a failed request to a channel
+func (s *Service) RecordFailure(channelID uint64) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	h := s.getOrCreateHealth(channelID)
+	h.consecutive++
+	h.lastCheck = time.Now()
+
+	if h.consecutive >= 3 {
+		h.status = store.ChannelHealthDegraded
+		h.cooldown = time.Now().Add(5 * time.Minute)
+	}
+	if h.consecutive >= 5 {
+		h.status = store.ChannelHealthCooldown
+		h.cooldown = time.Now().Add(15 * time.Minute)
+	}
+}
+
+// RecordTimeout records a timeout to a channel
+func (s *Service) RecordTimeout(channelID uint64) {
+	s.RecordFailure(channelID)
+}
+
+func (s *Service) getOrCreateHealth(channelID uint64) *channelHealth {
+	h, ok := s.healthStatus[channelID]
+	if !ok {
+		h = &channelHealth{
+			status: store.ChannelHealthHealthy,
+		}
+		s.healthStatus[channelID] = h
+	}
+	return h
+}
+
+func (s *Service) filterHealthy(channels []*store.Channel) []*store.Channel {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	var healthy []*store.Channel
+	now := time.Now()
+
+	for _, ch := range channels {
+		h, ok := s.healthStatus[ch.ID]
+		if !ok {
+			healthy = append(healthy, ch)
+			continue
+		}
+
+		// Check if cooldown has expired
+		if now.After(h.cooldown) && h.cooldown.IsZero() == false {
+			h.consecutive = 0
+			h.status = store.ChannelHealthHealthy
+			healthy = append(healthy, ch)
+			continue
+		}
+
+		if h.status == store.ChannelHealthHealthy || h.status == store.ChannelHealthDegraded {
+			healthy = append(healthy, ch)
+		}
+	}
+
+	return healthy
+}
+
+// GetHealthStatus returns the health status of a channel
+func (s *Service) GetHealthStatus(channelID uint64) string {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	h, ok := s.healthStatus[channelID]
+	if !ok {
+		return store.ChannelHealthHealthy
+	}
+	return h.status
+}
+
+// ChannelCandidate represents a channel with its resolved API key
+type ChannelCandidate struct {
+	Channel *store.Channel
+	APIKey  string
+	Format  string
+}
+
+// SelectCandidates returns candidates for a model, sorted by priority
+func (s *Service) SelectCandidates(ctx context.Context, modelName string, preferredFormat string) ([]ChannelCandidate, error) {
+	channels, err := s.channelDAO.GetEnabledChannelsByModel(modelName)
+	if err != nil {
+		return nil, err
+	}
+
+	var candidates []ChannelCandidate
+	for _, ch := range channels {
+		// Check if channel supports the preferred format
+		formats := ch.FormatsEffective()
+		supported := false
+		for _, f := range formats {
+			if f == preferredFormat || preferredFormat == "" {
+				supported = true
+				break
+			}
+		}
+		if !supported {
+			continue
+		}
+
+		apiKey, err := crypto.Decrypt(ch.APIKeyEnc)
+		if err != nil {
+			log.Printf("Failed to decrypt API key for channel %s: %v", ch.Name, err)
+			continue
+		}
+
+		candidates = append(candidates, ChannelCandidate{
+			Channel: ch,
+			APIKey:  apiKey,
+			Format:  preferredFormat,
+		})
+	}
+
+	return candidates, nil
+}