refactor: complete backend rewrite for multi-protocol proxy

Major rewrite of the Go backend to support:
- Three API format imports: openai, anthropic, compatible
- Three protocol conversions: Chat Completions, Responses, Messages
- Hub-and-spoke architecture with Chat as intermediate format

Deleted:
- opencat.go (old entry)
- store/, team/, pkg/team/, pkg/store/ (old data layer)
- internal/model/, internal/consts/ (old types)
- internal/service/team/, internal/controller/team/ (old handlers)
- llm/ (removed LLM client library, pure proxy mode)
- dist/, assets/ (old build artifacts)

Added:
- internal/store/ — 9 GORM models + multi-DB support
- internal/pkg/ — crypto (AES-GCM), apikey, jwt, ratelimit, resp, tokenizer
- internal/channel/ — channel selection, weighted LB, health checks
- internal/proxy/convert/ — 6 protocol conversion functions + SSE streaming
- internal/proxy/ — gateway with request dispatch and upstream selection
- internal/usage/ — async usage recorder with batch writes
- internal/api/ — management API (auth, users, keys, channels, models)
- Makefile for build/test/deploy

Fixed API to match frontend expectations:
- Login response wraps token in { data: { token } }
- GET /api/profile route added
- Profile response wraps user in { code, data }
- Role returned as number (10=admin, 1=user)

internal/channel/health.go

@@ -0,0 +1,108 @@
+package channel
+
+import (
+	"context"
+	"fmt"
+	"opencatd-open/internal/dao"
+	"opencatd-open/internal/store"
+	"opencatd-open/internal/pkg/crypto"
+	"net/http"
+	"time"
+)
+
+type HealthChecker struct {
+	channelDAO *dao.ChannelDAO
+	service    *Service
+	client     *http.Client
+}
+
+func NewHealthChecker(channelDAO *dao.ChannelDAO, service *Service) *HealthChecker {
+	return &HealthChecker{
+		channelDAO: channelDAO,
+		service:    service,
+		client: &http.Client{
+			Timeout: 10 * time.Second,
+		},
+	}
+}
+
+// CheckChannel performs a health check on a channel
+func (hc *HealthChecker) CheckChannel(ctx context.Context, channel *store.Channel) error {
+	apiKey, err := crypto.Decrypt(channel.APIKeyEnc)
+	if err != nil {
+		return fmt.Errorf("failed to decrypt API key: %w", err)
+	}
+
+	// Simple health check: try to list models
+	var url string
+	switch channel.Provider {
+	case store.ChannelProviderOpenAI:
+		url = channel.UpstreamURL("chat", "/models")
+	case store.ChannelProviderAnthropic:
+		url = "https://api.anthropic.com/v1/models"
+	default:
+		url = channel.UpstreamURL("chat", "/models")
+	}
+
+	req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
+	if err != nil {
+		return fmt.Errorf("failed to create request: %w", err)
+	}
+
+	// Set headers based on provider
+	switch channel.Provider {
+	case store.ChannelProviderOpenAI, store.ChannelProviderCompatible:
+		req.Header.Set("Authorization", "Bearer "+apiKey)
+	case store.ChannelProviderAnthropic:
+		req.Header.Set("x-api-key", apiKey)
+		req.Header.Set("anthropic-version", "2023-06-01")
+	}
+	req.Header.Set("Content-Type", "application/json")
+
+	resp, err := hc.client.Do(req)
+	if err != nil {
+		hc.service.RecordFailure(channel.ID)
+		return fmt.Errorf("health check failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode == http.StatusOK {
+		hc.service.RecordSuccess(channel.ID)
+		return nil
+	}
+
+	hc.service.RecordFailure(channel.ID)
+	return fmt.Errorf("health check returned status %d", resp.StatusCode)
+}
+
+// CheckAllChannels checks health of all enabled channels
+func (hc *HealthChecker) CheckAllChannels(ctx context.Context) error {
+	channels, err := hc.channelDAO.ListEnabled()
+	if err != nil {
+		return err
+	}
+
+	for _, ch := range channels {
+		if err := hc.CheckChannel(ctx, ch); err != nil {
+			fmt.Printf("Channel %s health check failed: %v\n", ch.Name, err)
+		}
+	}
+	return nil
+}
+
+// StartPeriodicCheck starts periodic health checks
+func (hc *HealthChecker) StartPeriodicCheck(ctx context.Context, interval time.Duration) {
+	ticker := time.NewTicker(interval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			if err := hc.CheckAllChannels(ctx); err != nil {
+				fmt.Printf("Periodic health check error: %v\n", err)
+			}
+		}
+	}
+}