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/auth.go

@@ -27,6 +27,7 @@ func (as *authServer) Login(_ context.Context, req *pb.LoginRequest) (*pb.LoginR
 	if t, err := time.Parse(time.RFC3339, expiresAtStr); err == nil {
 		expiresAt = timestamppb.New(t)
 	}
+	as.s.logger.Info("audit: login", "username", req.Username)
 	return &pb.LoginResponse{Token: token, ExpiresAt: expiresAt}, nil
 }
 
@@ -39,6 +40,7 @@ func (as *authServer) Logout(ctx context.Context, _ *pb.LogoutRequest) (*pb.Logo
 	if err == nil {
 		_ = as.s.auth.Logout(client)
 	}
+	as.s.logger.Info("audit: logout", "username", callerUsername(ctx))
 	return &pb.LogoutResponse{}, nil
 }