F-001/F-002/F-003: CRL prefix-scan, digest error sanitization, ctx-aware sleeps

F-001 (P3): GenerateDERCRL scoped to issuer via composite index
  - Add RevocationRepository.ListByIssuer leveraging migration 000012's
    idx_certificate_revocations_issuer_serial composite index as a
    prefix-scan target. Previously CAOperationsSvc.GenerateDERCRL called
    ListAll() and filtered by IssuerID in Go — O(total revocations)
    regardless of how many revocations belonged to the target issuer.
  - Rewrite GenerateDERCRL to call ListByIssuer(ctx, issuerID) so PostgreSQL
    drives a prefix scan of the composite index. Drops the in-memory filter.
  - New regression test in ca_operations_test.go asserts the CRL hot path
    invokes ListByIssuer exactly once and never ListAll, and that the
    issuerID is threaded through correctly.

F-002 (P3): digest.go admin-auth endpoints no longer leak internal errors
  - PreviewDigest (GET /api/v1/digest/preview) and SendDigest
    (POST /api/v1/digest/send) previously wrote err.Error() into the HTTP
    response body on 500s. Replace with slog.Error server-side logging plus
    a generic "internal error" response body, matching the house pattern
    in certificates.go and export.go.

F-003 (P4): three blocking time.Sleep sites now honor ctx cancellation
  - internal/connector/issuer/acme/acme.go:672 (DNS-01 propagation wait)
    now runs under a select{case <-ctx.Done(): CleanUp + return ctx.Err();
    case <-time.After(d):} so graceful shutdown doesn't get stuck behind
    the propagation delay.
  - internal/connector/issuer/acme/acme.go:786 (dns-persist-01 propagation
    wait) same pattern, returns ctx.Err() on cancel.
  - cmd/agent/main.go:272 (polling backoff inside the heartbeat loop) now
    wraps the sleep in select{case <-ctx.Done(): continue; case <-time.After(backoff):}
    so the outer <-ctx.Done() case on the parent loop fires cleanly.

Verification: build, vet, and race-enabled short tests green across all
55+ packages. govulncheck reports zero vulnerabilities in the code path.
No migration needed — F-001 reuses the existing 000012 composite index.
No frontend changes.

internal/service/testutil_test.go

@@ -1385,6 +1385,13 @@ type mockRevocationRepo struct {
 	Revocations []*domain.CertificateRevocation
 	CreateErr   error
 	ListErr     error
+	// F-001 regression instrumentation: track which list method was invoked
+	// so tests can assert that the CRL generation hot path uses the scoped
+	// ListByIssuer query (migration 000012 composite index) rather than
+	// ListAll followed by in-Go filtering.
+	ListAllCalls      int
+	ListByIssuerCalls int
+	LastListIssuerID  string
 }
 
 func (m *mockRevocationRepo) Create(ctx context.Context, revocation *domain.CertificateRevocation) error {
@@ -1405,12 +1412,28 @@ func (m *mockRevocationRepo) GetByIssuerAndSerial(ctx context.Context, issuerID,
 }
 
 func (m *mockRevocationRepo) ListAll(ctx context.Context) ([]*domain.CertificateRevocation, error) {
+	m.ListAllCalls++
 	if m.ListErr != nil {
 		return nil, m.ListErr
 	}
 	return m.Revocations, nil
 }
 
+func (m *mockRevocationRepo) ListByIssuer(ctx context.Context, issuerID string) ([]*domain.CertificateRevocation, error) {
+	m.ListByIssuerCalls++
+	m.LastListIssuerID = issuerID
+	if m.ListErr != nil {
+		return nil, m.ListErr
+	}
+	var result []*domain.CertificateRevocation
+	for _, r := range m.Revocations {
+		if r.IssuerID == issuerID {
+			result = append(result, r)
+		}
+	}
+	return result, nil
+}
+
 func (m *mockRevocationRepo) ListByCertificate(ctx context.Context, certID string) ([]*domain.CertificateRevocation, error) {
 	var result []*domain.CertificateRevocation
 	for _, r := range m.Revocations {