Add audit logging for all mutating gRPC operations

Log Info-level audit events on success for:
- system: Init, Unseal, Seal
- auth: Login, Logout
- engine: Mount, Unmount
- policy: CreatePolicy, DeletePolicy
- ca: ImportRoot, CreateIssuer, DeleteIssuer, IssueCert, RenewCert

Each log line includes relevant identifiers (mount, issuer, serial, CN,
SANs, username) so that certificate issuance and other privileged
operations are traceable in the server logs.

Co-authored-by: Junie <junie@jetbrains.com>

internal/grpcserver/engine.go

@@ -40,6 +40,12 @@ func (es *engineServer) Mount(ctx context.Context, req *pb.MountRequest) (*pb.Mo
 			return nil, status.Error(codes.Internal, err.Error())
 		}
 	}
+	ti := tokenInfoFromContext(ctx)
+	username := ""
+	if ti != nil {
+		username = ti.Username
+	}
+	es.s.logger.Info("audit: engine mounted", "name", req.Name, "type", req.Type, "username", username)
 	return &pb.MountResponse{}, nil
 }
 
@@ -53,6 +59,12 @@ func (es *engineServer) Unmount(ctx context.Context, req *pb.UnmountRequest) (*p
 		}
 		return nil, status.Error(codes.Internal, err.Error())
 	}
+	ti := tokenInfoFromContext(ctx)
+	username := ""
+	if ti != nil {
+		username = ti.Username
+	}
+	es.s.logger.Info("audit: engine unmounted", "name", req.Name, "username", username)
 	return &pb.UnmountResponse{}, nil
 }