Drop the compose file and systemd unit for the sync server

The server is a single process, so a compose file that only wrapped one
container added a second way to do the same thing. Docker is now the one
documented path, pointing at a Postgres that lives outside the container.

💘 Generated with Crush

Assisted-by: Crush:kimi-k3

README.md

@@ -58,8 +58,9 @@ PROMPT_SYNC_DATABASE_URL="postgres://localhost:5432/prompt_sync?sslmode=disable"
   go run .          # listens on :8080
 ```
 
-Or run it with Docker (`POSTGRES_PASSWORD=… docker compose up -d`) — see
-[Server/README.md](Server/README.md) for that and for deploying to a VPS.
+Or build the Docker image (`docker build -t prompt-sync Server/`) and point it
+at your own Postgres — see [Server/README.md](Server/README.md) for that and for
+deploying to a host.
 
 Then in the panel: click the sync icon, set the server URL (e.g.
 `http://192.168.1.10:8080` or an HTTPS host), and either **Generate Code** on