fix: address CodeRabbit review issues

- redis-handler: add TLS_CA and USERNAME env support matching core client
- connection.ts: fix Boolean() truthy bug (CI="false" was disabling Redis)
- channel.ts: add in-memory lock for single-instance fallback mode
- channel-subscription-tracker: lazy-init subscriber client
- board.ts: invalidate user cache on board delete for home-board users
- .env.example: document REDIS_CACHE_DB vs DATABASE_INDEX collision risk
- CLI docs: fix docker exec command (remove /bin/bash prefix)
- Docker docs: fix docker rm -f and docker ps -a for stopped containers

.env.example

@@ -56,7 +56,7 @@ TURBO_TELEMETRY_DISABLED=1
 # DOCKER_SOCKET_PATHS=
 
 # Redis DB index for the Next.js cache handler (default: 1).
-# Separate from the main pub/sub DB (0) to allow independent FLUSHDB.
+# Must differ from REDIS_DATABASE_INDEX (default: 0) to avoid cache ops affecting app data.
 # REDIS_CACHE_DB=1
 
 # Enable kubernetes tool