Fix SEC-11: use json.Marshal for audit details

- Add internal/audit package with JSON() and JSONWithRoles() helpers
  that use json.Marshal instead of fmt.Sprintf with %q
- Replace all fmt.Sprintf audit detail construction in:
  - internal/server/server.go (10 occurrences)
  - internal/ui/handlers_auth.go (4 occurrences)
  - internal/grpcserver/auth.go (4 occurrences)
- Add tests for the helpers including edge-case Unicode,
  null bytes, special characters, and odd argument counts
- Fix broken {"roles":%v} formatting that produced invalid JSON

Security: Audit log detail strings are now constructed via
json.Marshal, which correctly handles all Unicode edge cases
(U+2028, U+2029, null bytes, etc.) that fmt.Sprintf with %q
may mishandle. This prevents potential log injection or parsing
issues in audit event consumers.

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

internal/grpcserver/auth.go

@@ -13,6 +13,7 @@ import (
 	"google.golang.org/protobuf/types/known/timestamppb"
 
 	mciasv1 "git.wntrmute.dev/kyle/mcias/gen/mcias/v1"
+	"git.wntrmute.dev/kyle/mcias/internal/audit"
 	"git.wntrmute.dev/kyle/mcias/internal/auth"
 	"git.wntrmute.dev/kyle/mcias/internal/crypto"
 	"git.wntrmute.dev/kyle/mcias/internal/model"
@@ -42,7 +43,7 @@ func (a *authServiceServer) Login(ctx context.Context, req *mciasv1.LoginRequest
 		// Security: run dummy Argon2 to equalise timing for unknown users.
 		_, _ = auth.VerifyPassword("dummy", auth.DummyHash())
 		a.s.db.WriteAuditEvent(model.EventLoginFail, nil, nil, ip, //nolint:errcheck // audit failure is non-fatal
-			fmt.Sprintf(`{"username":%q,"reason":"unknown_user"}`, req.Username))
+			audit.JSON("username", req.Username, "reason", "unknown_user"))
 		return nil, status.Error(codes.Unauthenticated, "invalid credentials")
 	}
 
@@ -129,7 +130,7 @@ func (a *authServiceServer) Login(ctx context.Context, req *mciasv1.LoginRequest
 
 	a.s.db.WriteAuditEvent(model.EventLoginOK, &acct.ID, nil, ip, "") //nolint:errcheck
 	a.s.db.WriteAuditEvent(model.EventTokenIssued, &acct.ID, nil, ip, //nolint:errcheck
-		fmt.Sprintf(`{"jti":%q}`, claims.JTI))
+		audit.JSON("jti", claims.JTI))
 
 	return &mciasv1.LoginResponse{
 		Token:     tokenStr,
@@ -145,7 +146,7 @@ func (a *authServiceServer) Logout(ctx context.Context, _ *mciasv1.LogoutRequest
 		return nil, status.Error(codes.Internal, "internal error")
 	}
 	a.s.db.WriteAuditEvent(model.EventTokenRevoked, nil, nil, peerIP(ctx), //nolint:errcheck
-		fmt.Sprintf(`{"jti":%q,"reason":"logout"}`, claims.JTI))
+		audit.JSON("jti", claims.JTI, "reason", "logout"))
 	return &mciasv1.LogoutResponse{}, nil
 }
 
@@ -186,7 +187,7 @@ func (a *authServiceServer) RenewToken(ctx context.Context, _ *mciasv1.RenewToke
 	}
 
 	a.s.db.WriteAuditEvent(model.EventTokenRenewed, &acct.ID, nil, peerIP(ctx), //nolint:errcheck
-		fmt.Sprintf(`{"old_jti":%q,"new_jti":%q}`, claims.JTI, newClaims.JTI))
+		audit.JSON("old_jti", claims.JTI, "new_jti", newClaims.JTI))
 
 	return &mciasv1.RenewTokenResponse{
 		Token:     newTokenStr,