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.

.github/workflows/ci.yml

@@ -518,6 +518,42 @@ jobs:
           fi
           echo "B-1 orphan-CRUD client function guardrail: all 8 functions have page consumers."
 
+      - name: Forbidden strings.Contains(err.Error()) regression guard (S-2)
+        # S-2 closure (cat-s6-efc7f6f6bd50): replaced 30 brittle
+        # substring-match error-dispatch sites in internal/api/handler/
+        # with errors.Is + typed sentinels (repository.ErrNotFound,
+        # repository.ErrForeignKeyConstraint via the
+        # repository.IsForeignKeyError helper). This step grep-fails
+        # the build if any new strings.Contains(err.Error(), "not found")
+        # or strings.Contains(err.Error(), "violates foreign key")
+        # site appears under internal/api/handler/.
+        #
+        # Allowed: closure-comments documenting the convention (e.g.
+        # bulk_reassignment.go's "post-M-1 errToStatus convention"
+        # docblock); domain-specific substring patterns that are
+        # legitimately one-off ("cannot approve", "cannot reject",
+        # "cannot be parsed", "challenge password") — flagged as
+        # deferred follow-ups in the S-2 commit message.
+        #
+        # See coverage-gap-audit-2026-04-24-v5/unified-audit.md
+        # cat-s6-efc7f6f6bd50 for closure rationale.
+        run: |
+          set -e
+          BAD=$(grep -rnE 'strings\.Contains\(err\.Error\(\),\s*"(not found|violates foreign key|RESTRICT)"' internal/api/handler/ 2>/dev/null \
+              | grep -vE '^\s*[^:]+:[0-9]+:\s*//' \
+              || true)
+          if [ -n "$BAD" ]; then
+            echo "S-2 regression: brittle substring-match error-dispatch reappeared:"
+            echo "$BAD"
+            echo ""
+            echo "Use errors.Is(err, repository.ErrNotFound) for not-found dispatch,"
+            echo "or repository.IsForeignKeyError(err) for FK violations."
+            echo "See coverage-gap-audit-2026-04-24-v5/unified-audit.md"
+            echo "cat-s6-efc7f6f6bd50 for closure rationale."
+            exit 1
+          fi
+          echo "S-2 typed-sentinel error-dispatch guardrail: clean."
+
       - name: Race Detection
         run: go test -race ./internal/service/... ./internal/api/handler/... ./internal/api/middleware/... ./internal/scheduler/... ./internal/connector/... ./internal/crypto/... ./internal/domain/... ./internal/validation/... ./internal/tlsprobe/... -count=1 -timeout 300s