fix(bundle-5): Operational Liveness + Bootstrap — 4 audit findings closed

Closes Audit-2026-04-25 H-006 (High), H-007 (High), M-011 (Medium),
L-006 (Low — verified-already-closed via C-1 master closure in v2.0.54).
Hardens the orchestrator-facing surface — k8s probes, agent enrollment,
shutdown audit drain, scheduler config plumbing.

What changed
- internal/api/handler/health.go — split contract:
    * /health stays shallow 200 (k8s liveness — process alive)
    * /ready accepts *sql.DB; runs db.PingContext(2s); 503 on failure
    * Nil DB path returns 200 + db=not_configured (test fixtures)
- internal/api/handler/agent_bootstrap.go (NEW) — verifyBootstrapToken:
    * empty expected = warn-mode pass-through
    * non-empty = `Authorization: Bearer <token>` required
    * crypto/subtle.ConstantTimeCompare; length-mismatch path runs dummy
      compare to keep timing uniform
    * ErrBootstrapTokenInvalid sentinel
- internal/api/handler/agents.go — RegisterAgent calls verifyBootstrapToken
  BEFORE body parse so unauth probes don't even allocate a JSON decoder
- internal/config/config.go — two new env vars:
    * CERTCTL_AGENT_BOOTSTRAP_TOKEN  (Auth.AgentBootstrapToken)
    * CERTCTL_AUDIT_FLUSH_TIMEOUT_SECONDS (Server.AuditFlushTimeoutSeconds)
- cmd/server/main.go — 3 changes:
    * pass *sql.DB into NewHealthHandler (H-006)
    * pass cfg.Auth.AgentBootstrapToken into NewAgentHandler (H-007)
    * configurable shutdown audit-flush timeout (M-011)
    * one-shot startup WARN when bootstrap token unset (deprecation)
- new tests: agent_bootstrap_test.go (full deny/accept/warn-mode coverage,
  constant-time compare path, length-mismatch); health_test.go extended
  with /ready DB-probe failure (503), nil-DB pass-through, /health-shallow

L-006 verified
- cmd/server/main.go:557 already calls
  sched.SetShortLivedExpiryCheckInterval(cfg.Scheduler.ShortLivedExpiryCheckInterval)
  per the C-1 master closure in v2.0.54. Bundle 5 confirms; no code change.

Threat model: TB-1 (operator/orchestrator), TB-2 (Agent↔Server).
- CWE-754 (Improper Check for Unusual or Exceptional Conditions) for H-006
- CWE-306 + CWE-288 (Missing Authentication for Critical Function) for H-007

Verification
- go vet ./...                               → clean
- go build ./...                             → clean
- go test -short -count=1 ./...              → all packages pass
- targeted Bundle-5 regressions               → all pass
- npx tsc --noEmit (web)                     → clean
- npx vitest run (web)                       → in-flight (sandbox 45s
  ceiling exceeded; no failure markers in dot stream; no frontend
  changes in this bundle so no regression risk)
- python3 yaml.safe_load(api/openapi.yaml)   → 89 paths

Backward compatibility
- Bootstrap token defaults to empty (warn-mode) — existing demo
  deployments unaffected. Server logs deprecation WARN; v2.2.0 will
  require it.
- Audit flush timeout default 30s preserves prior behaviour.
- Helm chart already routes readiness probe to /ready (no chart change
  needed); now /ready actually probes the DB.

Bundle 5 of the 2026-04-25 comprehensive audit.

internal/integration/lifecycle_test.go

@@ -79,7 +79,7 @@ func TestCertificateLifecycle(t *testing.T) {
 	certificateHandler := handler.NewCertificateHandler(certificateService)
 	issuerHandler := handler.NewIssuerHandler(issuerService)
 	targetHandler := handler.NewTargetHandler(&mockTargetService{targetRepo: targetRepo, auditService: auditService})
-	agentHandler := handler.NewAgentHandler(agentService)
+	agentHandler := handler.NewAgentHandler(agentService, "") // Bundle-5 / H-007: integration fixture uses warn-mode pass-through
 	jobHandler := handler.NewJobHandler(jobService)
 	policyHandler := handler.NewPolicyHandler(policyService)
 	profileHandler := handler.NewProfileHandler(&mockProfileService{})
@@ -90,7 +90,7 @@ func TestCertificateLifecycle(t *testing.T) {
 	notificationHandler := handler.NewNotificationHandler(notificationService)
 	statsHandler := handler.NewStatsHandler(&mockStatsService{})
 	metricsHandler := handler.NewMetricsHandler(&mockStatsService{}, time.Now())
-	healthHandler := handler.NewHealthHandler("none")
+	healthHandler := handler.NewHealthHandler("none", nil) // Bundle-5 / H-006: integration fixture has no DB pool wired
 	discoveryHandler := handler.NewDiscoveryHandler(&mockDiscoveryService{})
 	networkScanHandler := handler.NewNetworkScanHandler(&mockNetworkScanService{})
 	verificationHandler := handler.NewVerificationHandler(&mockVerificationService{})