fix: security audit remediation (AUDIT-001, 003, 004, 005, 006, 018)

- AUDIT-001: Validate OpenSSL revoke inputs (hex-only serials, RFC 5280 reasons)
- AUDIT-003: Enforce /20 CIDR size cap at API level (create + update)
- AUDIT-004: Support comma-separated CERTCTL_AUTH_SECRET for zero-downtime key rotation
- AUDIT-005: Add ReadHeaderTimeout (5s) to prevent Slowloris
- AUDIT-006: Document audit trail query parameter exclusion rationale
- AUDIT-018: Add immediate-run-on-start to short-lived expiry scheduler loop

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/quickstart.md

@@ -84,6 +84,8 @@ curl http://localhost:8443/health
 Open **http://localhost:8443** in your browser.
 
 > **Note:** The Docker Compose demo runs with authentication disabled (`CERTCTL_AUTH_TYPE=none`) so you can explore immediately. For production, set `CERTCTL_AUTH_TYPE=api-key` and `CERTCTL_AUTH_SECRET=<your-secret>` in your environment, then pass `Authorization: Bearer <your-secret>` on all API requests. The dashboard will prompt for your API key on first load.
+>
+> **Key rotation:** `CERTCTL_AUTH_SECRET` accepts comma-separated keys (e.g., `CERTCTL_AUTH_SECRET=new-key,old-key`). Both keys are valid simultaneously, enabling zero-downtime rotation: add the new key, roll clients over, then remove the old key.
 
 The dashboard comes pre-loaded with 15 demo certificates across multiple teams, environments, and statuses — expiring certs, expired certs, active certs, failed renewals. A realistic snapshot of what certificate management looks like in a real organization.