feat: add metrics, observability, test fixtures, and scaling playbook artifacts

docs/DEPLOYMENT.md

@@ -8,6 +8,7 @@
   - [Production Target (Hetzner)](#production-target-hetzner)
     - [Planned Infrastructure](#planned-infrastructure)
     - [Cutover Plan](#cutover-plan)
+    - [Production Preflight](#production-preflight)
     - [Firewall / Security Group](#firewall--security-group)
     - [Pipeline Readiness](#pipeline-readiness)
   - [Target Topology](#target-topology)
@@ -64,6 +65,19 @@ Linear Void runs in two separate environments:
 6. Enable HTTPS, WAF, cache rules on production zone
 7. Decommission test lab as public target
 
+### Production Preflight
+
+Before the cutover in the plan above, treat the following as the minimum readiness gate:
+
+1. Verify the Hetzner host is reachable over SSH and Docker remote API TLS is working for the Gitea runner.
+2. Confirm all required production secrets are present under the `CI_PROD_*` prefix.
+3. Confirm `linearvoid.tv`, `www.linearvoid.tv`, and `media.linearvoid.tv` DNS targets are correct for the intended phase.
+4. Complete the manual Cloudflare/UptimeRobot checklist and attach evidence in `docs/operations/cloudflare-uptimerobot-checklist.md`.
+5. Run the production smoke path end to end: viewer SPA, HLS playback, chat connect, admin login, upload, playlist save, and playout health.
+6. Verify rollback is still possible before the final DNS switch.
+
+Readiness is not complete until the checklist above and the manual evidence tracker are both satisfied.
+
 ### Firewall / Security Group
 
 | Port | Source                | Purpose                        |