Fix ECDH zeroization, add audit logging, and remediate high findings

- Fix #61: handleRotateKey and handleDeleteUser now zeroize stored
  privBytes instead of calling Bytes() (which returns a copy). New
  state populates privBytes; old references nil'd for GC.
- Add audit logging subsystem (internal/audit) with structured event
  recording for cryptographic operations.
- Add audit log engine spec (engines/auditlog.md).
- Add ValidateName checks across all engines for path traversal (#48).
- Update AUDIT.md: all High findings resolved (0 open).
- Add REMEDIATION.md with detailed remediation tracking.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/grpcserver/engine.go

@@ -29,6 +29,14 @@ func (es *engineServer) Mount(ctx context.Context, req *pb.MountRequest) (*pb.Mo
 		}
 	}
 
+	// Inject external_url into engine config if available and not already set.
+	if config == nil {
+		config = make(map[string]interface{})
+	}
+	if _, ok := config["external_url"]; !ok && es.s.cfg.Server.ExternalURL != "" {
+		config["external_url"] = es.s.cfg.Server.ExternalURL
+	}
+
 	if err := es.s.engines.Mount(ctx, req.Name, engine.EngineType(req.Type), config); err != nil {
 		es.s.logger.Error("grpc: mount engine", "name", req.Name, "type", req.Type, "error", err)
 		switch {

internal/grpcserver/grpcserver_test.go

@@ -71,7 +71,7 @@ func newTestGRPCServer(t *testing.T) (*GRPCServer, func()) {
 		t.Fatalf("migrate: %v", err)
 	}
 	b := barrier.NewAESGCMBarrier(database)
-	sealMgr := seal.NewManager(database, b, slog.Default())
+	sealMgr := seal.NewManager(database, b, nil, slog.Default())
 	policyEngine := policy.NewEngine(b)
 	reg := newTestRegistry()
 	authenticator := auth.NewAuthenticator(nil, slog.Default())
@@ -82,7 +82,7 @@ func newTestGRPCServer(t *testing.T) (*GRPCServer, func()) {
 			Argon2Threads: 1,
 		},
 	}
-	srv := New(cfg, sealMgr, authenticator, policyEngine, reg, slog.Default())
+	srv := New(cfg, sealMgr, authenticator, policyEngine, reg, nil, slog.Default())
 	return srv, func() { _ = database.Close() }
 }
 

internal/grpcserver/interceptors.go

@@ -3,6 +3,7 @@ package grpcserver
 import (
 	"context"
 	"log/slog"
+	"path"
 	"strings"
 
 	"google.golang.org/grpc"
@@ -10,6 +11,7 @@ import (
 	"google.golang.org/grpc/metadata"
 	"google.golang.org/grpc/status"
 
+	"git.wntrmute.dev/kyle/metacrypt/internal/audit"
 	"git.wntrmute.dev/kyle/metacrypt/internal/auth"
 	"git.wntrmute.dev/kyle/metacrypt/internal/seal"
 )
@@ -97,6 +99,46 @@ func chainInterceptors(interceptors ...grpc.UnaryServerInterceptor) grpc.UnarySe
 	}
 }
 
+// auditInterceptor logs an audit event after each RPC completes. Must run
+// after authInterceptor so that caller info is available in the context.
+func auditInterceptor(auditLog *audit.Logger) grpc.UnaryServerInterceptor {
+	return func(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
+		resp, err := handler(ctx, req)
+
+		caller := "anonymous"
+		var roles []string
+		if ti := tokenInfoFromContext(ctx); ti != nil {
+			caller = ti.Username
+			roles = ti.Roles
+		}
+
+		outcome := "success"
+		var errMsg string
+		if err != nil {
+			outcome = "error"
+			if st, ok := status.FromError(err); ok {
+				if st.Code() == codes.PermissionDenied || st.Code() == codes.Unauthenticated {
+					outcome = "denied"
+				}
+				errMsg = st.Message()
+			} else {
+				errMsg = err.Error()
+			}
+		}
+
+		auditLog.Log(ctx, audit.Event{
+			Caller:    caller,
+			Roles:     roles,
+			Operation: path.Base(info.FullMethod),
+			Resource:  info.FullMethod,
+			Outcome:   outcome,
+			Error:     errMsg,
+		})
+
+		return resp, err
+	}
+}
+
 func extractToken(ctx context.Context) string {
 	md, ok := metadata.FromIncomingContext(ctx)
 	if !ok {

internal/grpcserver/server.go

@@ -13,6 +13,7 @@ import (
 
 	pb "git.wntrmute.dev/kyle/metacrypt/gen/metacrypt/v2"
 	internacme "git.wntrmute.dev/kyle/metacrypt/internal/acme"
+	"git.wntrmute.dev/kyle/metacrypt/internal/audit"
 	"git.wntrmute.dev/kyle/metacrypt/internal/auth"
 	"git.wntrmute.dev/kyle/metacrypt/internal/config"
 	"git.wntrmute.dev/kyle/metacrypt/internal/engine"
@@ -27,6 +28,7 @@ type GRPCServer struct {
 	auth         *auth.Authenticator
 	policy       *policy.Engine
 	engines      *engine.Registry
+	audit        *audit.Logger
 	logger       *slog.Logger
 	srv          *grpc.Server
 	acmeHandlers map[string]*internacme.Handler
@@ -35,13 +37,14 @@ type GRPCServer struct {
 
 // New creates a new GRPCServer.
 func New(cfg *config.Config, sealMgr *seal.Manager, authenticator *auth.Authenticator,
-	policyEngine *policy.Engine, engineRegistry *engine.Registry, logger *slog.Logger) *GRPCServer {
+	policyEngine *policy.Engine, engineRegistry *engine.Registry, auditLog *audit.Logger, logger *slog.Logger) *GRPCServer {
 	return &GRPCServer{
 		cfg:          cfg,
 		sealMgr:      sealMgr,
 		auth:         authenticator,
 		policy:       policyEngine,
 		engines:      engineRegistry,
+		audit:        auditLog,
 		logger:       logger,
 		acmeHandlers: make(map[string]*internacme.Handler),
 	}
@@ -68,6 +71,7 @@ func (s *GRPCServer) Start() error {
 		sealInterceptor(s.sealMgr, s.logger, sealRequiredMethods()),
 		authInterceptor(s.auth, s.logger, authRequiredMethods()),
 		adminInterceptor(s.logger, adminRequiredMethods()),
+		auditInterceptor(s.audit),
 	)
 
 	s.srv = grpc.NewServer(