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/pkg/jwt/jwt.go

@@ -0,0 +1,66 @@
+package jwt
+
+import (
+	"errors"
+	"strconv"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+// Claims is the payload of a signed token.
+type Claims struct {
+	UserID   int64  `json:"uid"`
+	Username string `json:"username"`
+	Role     string `json:"role"`
+	Type     string `json:"typ"` // access | refresh
+	jwt.RegisteredClaims
+}
+
+func sign(secret string, c Claims) (string, error) {
+	t := jwt.NewWithClaims(jwt.SigningMethodHS256, c)
+	return t.SignedString([]byte(secret))
+}
+
+// SignAccess issues a short-lived access token.
+func SignAccess(secret string, userID int64, username, role string, ttl time.Duration) (string, error) {
+	return sign(secret, Claims{
+		UserID: userID, Username: username, Role: role, Type: "access",
+		RegisteredClaims: jwt.RegisteredClaims{
+			ExpiresAt: jwt.NewNumericDate(time.Now().Add(ttl)),
+			IssuedAt:  jwt.NewNumericDate(time.Now()),
+			Subject:   fmtID(userID),
+		},
+	})
+}
+
+// SignRefresh issues a long-lived refresh token.
+func SignRefresh(secret string, userID int64, ttl time.Duration) (string, error) {
+	return sign(secret, Claims{
+		UserID: userID, Type: "refresh",
+		RegisteredClaims: jwt.RegisteredClaims{
+			ExpiresAt: jwt.NewNumericDate(time.Now().Add(ttl)),
+			IssuedAt:  jwt.NewNumericDate(time.Now()),
+			Subject:   fmtID(userID),
+		},
+	})
+}
+
+// Parse validates a token and returns its claims.
+func Parse(secret, token string) (*Claims, error) {
+	var c Claims
+	parsed, err := jwt.ParseWithClaims(token, &c, func(t *jwt.Token) (interface{}, error) {
+		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, errors.New("unexpected signing method")
+		}
+		return []byte(secret), nil
+	})
+	if err != nil || !parsed.Valid {
+		return nil, errors.New("invalid token")
+	}
+	return &c, nil
+}
+
+func fmtID(id int64) string {
+	return strconv.FormatInt(id, 10)
+}