Replace DragonflyDB with Valkey throughout codebase

- Update all DragonflyDB references to Valkey in README and docker-compose.yml
- Update install instructions to use Valkey package repository and .deb download
- Update configuration paths from /etc/dragonfly/ to /etc/valkey/
- Update version requirement to Valkey 8.x+ (successor to Redis OSS)

docker-compose.yml

@@ -12,7 +12,7 @@ services:
       network: host
     container_name: epicnext-cms
     # Runs on the host network so existing 127.0.0.1 refs in .env keep working:
-    # MariaDB (3306), DragonflyDB/Redis (6379), emulator RCON (3003) + API (3001),
+    # MariaDB (3306), Valkey/Redis (6379), emulator RCON (3003) + API (3001),
     # and the imaging renderer (8082). The container then listens directly on the
     # host's 3002 (the same port the current host-side CMS uses).
     # NOTE: stop the host CMS (next-server on 3002) first, otherwise the port is taken.