security: scope revocation unique index to (issuer_id, serial_number) (fixes H-1)

RFC 5280 §5.2.3 defines certificate serial number uniqueness per issuing CA,
not globally. The prior unique index on `certificate_revocations.serial_number`
enforced a stricter invariant than the spec: with 12 issuer connectors (Local
CA, ACME, Vault, step-ca, OpenSSL, DigiCert, Sectigo, Google CAS, AWS ACM PCA,
Entrust, GlobalSign, EJBCA), two distinct certificates legitimately issued by
different CAs can share a serial number. Recording a revocation for the second
collision silently dropped via `ON CONFLICT DO NOTHING`, leaving the second
cert persistently absent from OCSP/CRL responses.

Changes:

- Migration 000012 drops `idx_certificate_revocations_serial` and creates
  `idx_certificate_revocations_issuer_serial` UNIQUE ON (issuer_id,
  serial_number). Adds a non-unique `idx_certificate_revocations_serial_lookup`
  to preserve the serial-only fast path for OCSP/CRL probes that already know
  the issuer scope.
- `CertificateRevocationRepository.Create` targets the new composite key in
  `ON CONFLICT` — same-issuer idempotency preserved, cross-issuer collisions
  now recorded as distinct rows.
- `GetBySerial(serial)` renamed `GetByIssuerAndSerial(issuerID, serial)` on
  the interface and Postgres impl. All callers (OCSP responder, CRL
  generator, short-lived-cert exemption check) already have `issuerID` in
  scope because the protocol paths carry it (`/api/v1/ocsp/{issuer_id}/{serial}`,
  `/api/v1/crl/{issuer_id}`).
- Repository integration test added: `TestRevocationRepository_CrossIssuerSerialCollision`
  asserts that serial `CAFEBABE01` can be stored under two issuers
  simultaneously, that lookups return the correct row per (issuer, serial),
  and that same-issuer idempotency still works (re-inserting (issuer, serial)
  does not error and does not duplicate).
- Existing tests and service/integration mocks updated for the rename.

Wire-format invariants preserved: CRL DER bytes, OCSP response bytes, and
AES-256-GCM config encryption are unaffected — this change touches only
revocation-record uniqueness scope.

CWE-664.

internal/repository/interfaces.go

@@ -31,10 +31,15 @@ type CertificateRepository interface {
 
 // RevocationRepository defines operations for managing certificate revocations.
 type RevocationRepository interface {
-	// Create records a new certificate revocation.
+	// Create records a new certificate revocation. Uniqueness is scoped to
+	// (issuer_id, serial_number) per RFC 5280 §5.2.3, so duplicate serials
+	// across different issuers are permitted.
 	Create(ctx context.Context, revocation *domain.CertificateRevocation) error
-	// GetBySerial retrieves a revocation by serial number.
-	GetBySerial(ctx context.Context, serial string) (*domain.CertificateRevocation, error)
+	// GetByIssuerAndSerial retrieves a revocation by the (issuer_id, serial_number)
+	// pair. Callers (OCSP, CRL generation) always know the issuer because
+	// protocol endpoints carry it in the request path; RFC 5280 §5.2.3 guarantees
+	// uniqueness only within a single issuer.
+	GetByIssuerAndSerial(ctx context.Context, issuerID, serial string) (*domain.CertificateRevocation, error)
 	// ListAll returns all revocations, ordered by revocation time (for CRL generation).
 	ListAll(ctx context.Context) ([]*domain.CertificateRevocation, error)
 	// ListByCertificate returns all revocations for a certificate.