Close M-004 (OCSP issuer binding) and M-005 (discovery actor propagation) coverage-gap findings

M-004 — OCSP issuer binding (composite key):
  The OCSP lookup path now binds (issuer_id, serial) as a composite key
  rather than resolving by serial alone. CertificateRepository and
  RevocationRepository gain GetByIssuerAndSerial methods; ca_operations.go
  scopes both lookups by the issuer_id path param. When no managed cert
  binds to that (issuer, serial) tuple, GetOCSPResponse constructs an
  RFC 6960 §2.2 'unknown' response (CertStatus=2) instead of the prior
  default 'good'. Short-lived cert exemption (profile TTL < 1h) is
  preserved. Real repo errors (non-sql.ErrNoRows) fail closed with a log.

  Regression coverage: internal/service/ca_operations_test.go
    - TestCAOperationsSvc_GetOCSPResponse_Unknown_CrossIssuer
    - TestCAOperationsSvc_GetOCSPResponse_Unknown_UnknownSerial

M-005 — Discovery Claim/Dismiss actor propagation:
  DiscoveryService.ClaimDiscovered and DismissDiscovered now accept an
  explicit 'actor string' parameter (propagation pattern mirrors
  bulk_revocation.go / revocation_svc.go). The handler layer passes
  resolveActor(r.Context()) — the named-key identity established by the
  M-002 auth unification — and the service falls back to 'api' (the same
  safe sentinel resolveActor uses when no auth context is present) only
  when the caller passes an empty string. Never falls back to 'operator'.

  Regression coverage: internal/service/discovery_test.go
    - TestDiscoveryService_ClaimDiscovered_AuditActor
    - TestDiscoveryService_DismissDiscovered_AuditActor
    - TestDiscoveryService_ClaimDiscovered_EmptyActorFallsBackToAPI
    - TestDiscoveryService_DismissDiscovered_EmptyActorFallsBackToAPI

Each new test asserts event.Actor matches the caller-supplied string (or
'api' on empty input) and explicitly asserts event.Actor != 'operator'
to lock in the historical fix intent.

Files:
  internal/api/handler/discovery.go          — pass resolveActor(ctx)
  internal/api/handler/discovery_handler_test.go — updated call sites
  internal/integration/lifecycle_test.go     — updated mock wiring
  internal/repository/interfaces.go          — GetByIssuerAndSerial on
                                               CertificateRepository +
                                               RevocationRepository
  internal/repository/postgres/certificate.go — composite key lookup
  internal/service/ca_operations.go          — (issuer_id, serial) scoping
  internal/service/ca_operations_test.go     — 2 new M-004 tests
  internal/service/discovery.go              — actor parameter + 'api' fallback
  internal/service/discovery_test.go         — 4 new M-005 tests
  internal/service/shortlived_test.go        — mock signature update
  internal/service/testutil_test.go          — mock GetByIssuerAndSerial

internal/repository/postgres/certificate.go

@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"encoding/base64"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"strings"
 	"time"
@@ -272,6 +273,38 @@ func (r *CertificateRepository) Get(ctx context.Context, id string) (*domain.Man
 	return cert, nil
 }
 
+// GetByIssuerAndSerial retrieves a certificate by the (issuer_id, serial_number)
+// pair via a JOIN on certificate_versions. Per RFC 5280 §5.2.3, serial numbers
+// are unique only within a single issuer — callers that know the issuer (OCSP,
+// CRL generation, revocation lookup) use this method to scope lookups
+// correctly. Returns sql.ErrNoRows when no match exists so callers can
+// distinguish "unknown cert" (return OCSP status unknown) from a real
+// repository error.
+func (r *CertificateRepository) GetByIssuerAndSerial(ctx context.Context, issuerID, serial string) (*domain.ManagedCertificate, error) {
+	row := r.db.QueryRowContext(ctx, `
+		SELECT mc.id, mc.name, mc.common_name, mc.sans, mc.environment, mc.owner_id, mc.team_id,
+		       mc.issuer_id, mc.renewal_policy_id, mc.certificate_profile_id, mc.status, mc.expires_at,
+		       mc.tags, mc.last_renewal_at, mc.last_deployment_at, mc.revoked_at, mc.revocation_reason,
+		       mc.created_at, mc.updated_at
+		FROM managed_certificates mc
+		JOIN certificate_versions cv ON cv.certificate_id = mc.id
+		WHERE mc.issuer_id = $1 AND cv.serial_number = $2
+		LIMIT 1
+	`, issuerID, serial)
+
+	cert, err := r.scanCertificate(ctx, row)
+	if err != nil {
+		// scanCertificate wraps sql.ErrNoRows via %w, so surface the bare
+		// sentinel here for callers that branch on it with errors.Is.
+		if errors.Is(err, sql.ErrNoRows) {
+			return nil, sql.ErrNoRows
+		}
+		return nil, fmt.Errorf("failed to query certificate by issuer+serial: %w", err)
+	}
+
+	return cert, nil
+}
+
 // Create stores a new certificate
 func (r *CertificateRepository) Create(ctx context.Context, cert *domain.ManagedCertificate) error {
 	if cert.ID == "" {