server: main exits before graceful shutdown (drain, WAL checkpoint, dbClose) completes #577

Found in the #571 review (Minimax). This is pre-existing and was not changed by #571.

cmd/server/main.go: main returns as soon as ListenAndServe returns ErrServerClosed. That happens at the start of httpServer.Shutdown, and main does not wait for the shutdown goroutine. As a result, the client drain, the WAL checkpoint and dbClose race process exit. The deferred stopPingScoreHistory happens to buy time.

Fix: main waits on a done channel from the shutdown goroutine, with a bounded timeout. Add a process test asserting that "shutdown complete" is logged and that the DB is closed cleanly on SIGTERM.

Relates to #571.