chore: move feature list from TODO to BACKLOG and reorg admin/chat analytics items

.github/instructions/BACKLOG.md

@@ -171,3 +171,12 @@
 - [ ] Capacity triggers: Prometheus rules for cache-miss >10%/15min, CPU >80%/10min
 - [ ] Plan Phase 2 service separation — document boundaries (playout CPU, chat latency, DB state, monitoring), estimate per-service resources
 - [ ] Plan Phase 3 multi-origin resilience — research second Hetzner location, design warm standby + DNS failover + DB/Redis replication
+
+## Feature List (Future Enhancements)
+
+- [ ] Admin backend
+  - [ ] Analytics: implement a settings entry to set matomo url and page id
+- [ ] User frontend
+  - [ ] Chat: add user registration and login flows
+  - [ ] Chat: private DM history and search
+  - [ ] Analytics: add javascript snippet to send pageview events to matomo