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

docs/architecture/adr/ADR-003-tech-stack.md

@@ -38,7 +38,7 @@ We must explicitly document language/runtime choices per module and the assumpti
 
 1. All video assets **must** be normalized before being added to any playlist (H.264, AAC, 30 fps constant, 2s keyframes, CRF encoding).
 2. Deployment is **Docker-first**; services will be containerized.
-3. Cloudflare is the primary CDN and will be configured with specific Cache Rules (`.ts` immutable long TTL, `.m3u8` short TTL).
+3. Cloudflare is the primary CDN and will be configured with specific [Cache Rules](../../../infrastructure/cloudflare/cache-rules.md).
 4. Chat does **not** require end-to-end encryption in v1 (separate ADR if needed).
 5. Single VPS origin initially; multi-region is a later evolution.
 6. Monitoring focuses on playout health, cache hit ratio, and R2 operation counts.