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/renewal_policy.go

@@ -1,6 +1,7 @@
 package handler
 
 import (
+	"github.com/shankar0123/certctl/internal/repository"
 	"context"
 	"encoding/json"
 	"errors"
@@ -26,14 +27,14 @@ type RenewalPolicyService interface {
 
 // RenewalPolicyHandler serves /api/v1/renewal-policies CRUD endpoints.
 //
-// G-1 design note: the service-level `ErrRenewalPolicyDuplicateName` /
+// G-1 + S-2 design note: the service-level `ErrRenewalPolicyDuplicateName` /
 // `ErrRenewalPolicyInUse` sentinels alias the repository sentinels (same var
-// identity), so `errors.Is` walks transparently across layers. Delete/Update
-// not-found detection intentionally uses a `strings.Contains(err.Error(),
-// "not found")` substring check — the repo wraps `sql.ErrNoRows` as
-// `fmt.Errorf("renewal policy not found: %s", id)` which strips the sentinel,
-// and the handler red-tests' `ErrMockNotFound = errors.New("mock not found
-// error")` follows the same substring convention.
+// identity), so `errors.Is` walks transparently across layers. S-2 closure
+// (cat-s6-efc7f6f6bd50) extends the same convention to not-found detection:
+// repos now wrap `sql.ErrNoRows` via `fmt.Errorf("X not found: %w",
+// repository.ErrNotFound)`, handler dispatch uses
+// `errors.Is(err, repository.ErrNotFound)`, and `ErrMockNotFound` in
+// test_utils.go wraps the same sentinel so the mocks still resolve to 404.
 type RenewalPolicyHandler struct {
 	svc RenewalPolicyService
 }
@@ -191,7 +192,7 @@ func (h RenewalPolicyHandler) UpdateRenewalPolicy(w http.ResponseWriter, r *http
 			ErrorWithRequestID(w, http.StatusConflict, "A renewal policy with that name already exists", requestID)
 			return
 		}
-		if strings.Contains(err.Error(), "not found") {
+		if errors.Is(err, repository.ErrNotFound) {
 			ErrorWithRequestID(w, http.StatusNotFound, "Renewal policy not found", requestID)
 			return
 		}
@@ -231,7 +232,7 @@ func (h RenewalPolicyHandler) DeleteRenewalPolicy(w http.ResponseWriter, r *http
 			ErrorWithRequestID(w, http.StatusConflict, "Renewal policy is still referenced by managed certificates", requestID)
 			return
 		}
-		if strings.Contains(err.Error(), "not found") {
+		if errors.Is(err, repository.ErrNotFound) {
 			ErrorWithRequestID(w, http.StatusNotFound, "Renewal policy not found", requestID)
 			return
 		}