Replace token-based admin auth with JWT session authentication

- Add AdminUser model (bcrypt hashed passwords) and admin_users table
- Add AdminJWTService for HS256 JWT sessions (24h TTL)
- Add AdminSessionAuth middleware for /api/v1/admin/* routes
- Add admin handlers: login, logout, me, change-password, users CRUD
- Keys and model management routes now require admin JWT session
- Remove ADMIN_TOKEN, add ADMIN_USERNAME, ADMIN_PASSWORD env vars
- Update frontend: username/password login, admin_session storage, AdminUsers CRUD view

internal/db/seed.go

@@ -1,42 +1,59 @@
 package db
 
 import (
+	"crypto/rand"
+	"encoding/hex"
 	"log/slog"
+	"strings"
 
 	"golang.org/x/crypto/bcrypt"
 	"github.com/google/uuid"
 	"gorm.io/gorm"
 )
 
-func SeedAdminKey(db *gorm.DB, adminToken string) error {
-	var count int64
-	db.Model(&ApiKey{}).Count(&count)
+func generateRandomPassword(length int) (string, error) {
+	bytes := make([]byte, length)
+	if _, err := rand.Read(bytes); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(bytes)[:length], nil
+}
 
+func SeedAdminUser(db *gorm.DB, username, password string) error {
+	var count int64
+	db.Model(&AdminUser{}).Count(&count)
 	if count > 0 {
-		slog.Info("admin key already exists, skipping seed")
+		slog.Info("admin user already exists, skipping seed")
 		return nil
 	}
 
-	hash, err := bcrypt.GenerateFromPassword([]byte(adminToken), bcrypt.DefaultCost)
+	pwd := password
+	if pwd == "" {
+		var err error
+		pwd, err = generateRandomPassword(24)
+		if err != nil {
+			return err
+		}
+		slog.Warn("no ADMIN_PASSWORD set — generated random password (save this, it won't be shown again)")
+	}
+
+	hash, err := bcrypt.GenerateFromPassword([]byte(pwd), bcrypt.DefaultCost)
 	if err != nil {
 		return err
 	}
 
-	admin := ApiKey{
-		ID:        uuid.New(),
-		Name:      "admin",
-		KeyHash:   string(hash),
-		KeyPrefix: "admin-",
-		Scopes:    StringArray{"chat", "models", "usage", "admin"},
-		IsActive:  true,
-		IsAdmin:   true,
+	admin := AdminUser{
+		ID:           uuid.New(),
+		Username:     strings.ToLower(username),
+		PasswordHash: string(hash),
+		IsActive:    true,
 	}
 
 	if err := db.Create(&admin).Error; err != nil {
 		return err
 	}
 
-	slog.Info("admin key seeded", "prefix", admin.KeyPrefix)
-	slog.Warn("ADMIN TOKEN: " + adminToken + " (save this!)")
+	slog.Info("admin user seeded", "username", admin.Username)
+	slog.Warn("ADMIN USER: username=" + username + " password=" + pwd + " (save this, it won't be shown again)")
 	return nil
 }