refactor: modularize chat backend, update docs for .m4s segments

CHANGELOG.md

@@ -16,10 +16,15 @@
 - **CI pipeline**: Removed redundant `release` job (handled by `release.yml`).
 - **Tag format**: Standardized on `*.*.*` (e.g. `1.0.9`), removed `v` prefix from existing tags.
 - **DEPLOYMENT.md**: Documented parallel CI/CD pipeline flow with workflow table.
+- **Runtime checks**: Executed normalization edge-case tests (MOV, MKV, VFR, 5.1 input) and concat continuity verification in running playout environment.
+- **Incident runbook**: Added quarterly disaster recovery drill procedure with prerequisites, outage simulation, restore validation, and evidence template.
+- **Manual ops checklist**: Added `docs/operations/cloudflare-uptimerobot-checklist.md` to execute and record Cloudflare + UptimeRobot dashboard tasks with evidence capture.
+- **Scaling playbook**: Added `docs/operations/scaling-performance-playbook.md` with benchmark methods, trigger thresholds, and Phase 2/3 scaling decision flow.
 
 ### Fixed
 
 - **Prometheus config mount**: Switched prod from bind mounts to named volume `prometheus-config`, populated via `docker run` on remote host. Fixes `not a directory` error when bind source path doesn't exist on deployment host.
+- **Viewer mobile overflow**: Replaced `100vw` with `100%` on full-screen viewer containers to remove horizontal overflow on mobile viewports.
 
 ### Removed