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/pkg/jwt/jwt.go

@@ -0,0 +1,61 @@
+package jwt
+
+import (
+	"errors"
+	"time"
+
+	gojwt "github.com/golang-jwt/jwt/v5"
+)
+
+type Claims struct {
+	UserID uint64 `json:"user_id"`
+	Name   string `json:"name"`
+	Role   string `json:"role"`
+	gojwt.RegisteredClaims
+}
+
+// GenerateTokenPair 生成 access + refresh token
+func GenerateTokenPair(userID uint64, name, role, secret string, accessExpire, refreshExpire time.Duration) (accessToken, refreshToken string, err error) {
+	accessToken, err = generateToken(userID, name, role, "access", secret, accessExpire)
+	if err != nil {
+		return "", "", err
+	}
+	refreshToken, err = generateToken(userID, name, role, "refresh", secret, refreshExpire)
+	if err != nil {
+		return "", "", err
+	}
+	return
+}
+
+func generateToken(userID uint64, name, role, tokenType, secret string, expire time.Duration) (string, error) {
+	now := time.Now()
+	claims := Claims{
+		UserID: userID,
+		Name:   name,
+		Role:   role,
+		RegisteredClaims: gojwt.RegisteredClaims{
+			ExpiresAt: gojwt.NewNumericDate(now.Add(expire)),
+			IssuedAt:  gojwt.NewNumericDate(now),
+			NotBefore: gojwt.NewNumericDate(now),
+		},
+	}
+	token := gojwt.NewWithClaims(gojwt.SigningMethodHS256, claims)
+	return token.SignedString([]byte(secret))
+}
+
+// ValidateToken 校验 JWT
+func ValidateToken(tokenString, secret string) (*Claims, error) {
+	token, err := gojwt.ParseWithClaims(tokenString, &Claims{}, func(token *gojwt.Token) (interface{}, error) {
+		if _, ok := token.Method.(*gojwt.SigningMethodHMAC); !ok {
+			return nil, errors.New("unexpected signing method")
+		}
+		return []byte(secret), nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	if claims, ok := token.Claims.(*Claims); ok && token.Valid {
+		return claims, nil
+	}
+	return nil, gojwt.ErrInvalidKey
+}