refactor(handler,repo): replace strings.Contains error dispatch with typed sentinels (S-2)

Closes one 2026-04-24 audit finding (P2):

  - cat-s6-efc7f6f6bd50: 30 strings.Contains(err.Error(), ...) sites
    in internal/api/handler/ — brittle to repository-layer message
    changes, untyped against the actual failure mode.

Approach (Option B from prompt design notes):
  - New typed sentinels in internal/repository/errors.go:
      ErrNotFound, ErrForeignKeyConstraint
      IsForeignKeyError(err) helper (the only place substring
      matching at the lib/pq boundary is allowed; isolates the
      DB-driver string knowledge to one function).
  - New typed sentinel in internal/domain/errors.go:
      ErrValidation (reserved for future per-entity validation
      wrappers; not yet used by all handlers).
  - 49 sites in internal/repository/postgres/*.go updated to wrap
    sql.ErrNoRows-derived errors via fmt.Errorf("...: %w",
    repository.ErrNotFound).
  - 18 not-found handler sites + 2 FK-constraint handler sites
    refactored to errors.Is(err, repository.ErrNotFound) /
    repository.IsForeignKeyError(err).
  - 23 inline `fmt.Errorf("X not found")` test fixtures across
    handler tests rewrapped to wrap repository.ErrNotFound.
  - test_utils.go::ErrMockNotFound rewrapped to wrap
    repository.ErrNotFound; renewal_policy.go closure docblock
    updated to reflect the new convention.
  - integration test mockJobRepository.Get wraps repository.ErrNotFound.

CI regression guardrail:
- .github/workflows/ci.yml::"Forbidden strings.Contains(err.Error())
  regression guard (S-2)" greps for the three patterns ("not found",
  "violates foreign key", "RESTRICT") under internal/api/handler/
  and fails the build on regression.

Verification:
- go build ./... — clean
- go vet ./... — clean
- go test ./... -short -count=1 — all packages pass (handler +
  repository + service + integration)
- golangci-lint v2.11.4 run ./... — 0 issues
- S-2 guardrail dry-run on post-fix tree → empty (good)
- All sibling guardrails (S-1, G-3, D-1+D-2, B-1, L-1, H-1, C-1, F-1, P-1) pass

Audit findings closed:
- cat-s6-efc7f6f6bd50 (P2)

Deferred follow-ups:
- 6 domain-specific substring patterns still inline in handlers
  ("cannot approve", "cannot reject", "cannot be parsed",
  "no certificates found", "challenge password", "invalid"/
  "required" validation chains in profiles + agent_groups). Each
  needs its own typed sentinel, scoped per service. Documented
  by the S-2 CI guardrail's allowlist for closure-comments only.
- Per-entity not-found sentinels (Option A — ErrCertificateNotFound,
  ErrAgentNotFound, etc.) deferred. Generic ErrNotFound covers the
  current dispatch needs; per-entity precision would let handlers
  return entity-aware error bodies without a domain.Type field,
  but not blocking.

internal/api/handler/certificate_handler_test.go

@@ -900,7 +900,7 @@ func TestRevokeCertificate_Handler_AlreadyRevoked(t *testing.T) {
 func TestRevokeCertificate_Handler_NotFound(t *testing.T) {
 	mock := &MockCertificateService{
 		RevokeCertificateFn: func(_ context.Context, certID string, reason string, _ string) error {
-			return fmt.Errorf("failed to fetch certificate: not found")
+			return fmt.Errorf("failed to fetch certificate: not found: %w", ErrMockNotFound)
 		},
 	}
 
@@ -1033,7 +1033,7 @@ func TestGetDERCRL_Success(t *testing.T) {
 			if issuerID == "iss-local" {
 				return derCRLData, nil
 			}
-			return nil, fmt.Errorf("issuer not found")
+			return nil, fmt.Errorf("issuer not found: %w", ErrMockNotFound)
 		},
 	}
 
@@ -1061,7 +1061,7 @@ func TestGetDERCRL_Success(t *testing.T) {
 func TestGetDERCRL_IssuerNotFound(t *testing.T) {
 	mock := &MockCertificateService{
 		GenerateDERCRLFn: func(_ context.Context, issuerID string) ([]byte, error) {
-			return nil, fmt.Errorf("issuer not found")
+			return nil, fmt.Errorf("issuer not found: %w", ErrMockNotFound)
 		},
 	}
 
@@ -1118,7 +1118,7 @@ func TestHandleOCSP_Success(t *testing.T) {
 			if issuerID == "iss-local" && serialHex == "12345" {
 				return ocspResponseBytes, nil
 			}
-			return nil, fmt.Errorf("certificate not found")
+			return nil, fmt.Errorf("certificate not found: %w", ErrMockNotFound)
 		},
 	}
 
@@ -1159,7 +1159,7 @@ func TestHandleOCSP_MissingSerial(t *testing.T) {
 func TestHandleOCSP_IssuerNotFound(t *testing.T) {
 	mock := &MockCertificateService{
 		GetOCSPResponseFn: func(_ context.Context, issuerID string, serialHex string) ([]byte, error) {
-			return nil, fmt.Errorf("issuer not found")
+			return nil, fmt.Errorf("issuer not found: %w", ErrMockNotFound)
 		},
 	}
 
@@ -1178,7 +1178,7 @@ func TestHandleOCSP_IssuerNotFound(t *testing.T) {
 func TestHandleOCSP_CertNotFound(t *testing.T) {
 	mock := &MockCertificateService{
 		GetOCSPResponseFn: func(_ context.Context, issuerID string, serialHex string) ([]byte, error) {
-			return nil, fmt.Errorf("certificate not found")
+			return nil, fmt.Errorf("certificate not found: %w", ErrMockNotFound)
 		},
 	}
 
@@ -1529,7 +1529,7 @@ func TestGetCertificateDeployments_Success(t *testing.T) {
 func TestGetCertificateDeployments_NotFound(t *testing.T) {
 	mock := &MockCertificateService{
 		GetCertificateDeploymentsFn: func(_ context.Context, certID string) ([]domain.DeploymentTarget, error) {
-			return nil, fmt.Errorf("certificate not found")
+			return nil, fmt.Errorf("certificate not found: %w", ErrMockNotFound)
 		},
 	}