fix(backend): bootstrap admin from env instead of seeding admin/admin

007_seed_data.sql shipped a fixed admin account whose bcrypt hash decodes
to the password "admin", giving every deployment the same known
credentials. The seed row is removed; UserService.EnsureAdmin now creates
the administrator on startup from ADMIN_USERNAME / ADMIN_PASSWORD. It is
idempotent and never overwrites an existing password, so an operator who
rotates the admin password keeps it across restarts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/cmd/server/main.go

@@ -94,6 +94,12 @@ func main() {
 	)
 	userSvc := service.NewUserService(userRepo, auditSvc)
 
+	// Bootstrap the initial administrator (idempotent).
+	if err := userSvc.EnsureAdmin(context.Background(), cfg.AdminUsername, cfg.AdminPassword); err != nil {
+		slog.Error("failed to bootstrap admin user", "err", err)
+		os.Exit(1)
+	}
+
 	// Handlers
 	authMiddleware  := handler.NewAuthMiddleware(authSvc)
 	authHandler     := handler.NewAuthHandler(authSvc)