Poll sync cheaply and make the server deployable

Polling re-downloaded the whole store, photos included, every five
seconds. The app now checks a tiny version endpoint and only fetches the
payload when it actually moved, and the server gained request logging,
graceful shutdown, and self-hosting docs so it can run on a VPS instead
of only on localhost.

💘 Generated with Crush

Assisted-by: Crush:kimi-k3

README.md

@@ -66,9 +66,11 @@ Notes:
 
 - Both Macs must be able to reach the server. A LAN address or a Tailscale
   hostname works; `localhost` only works if the server runs on the same Mac.
+  See [Server/README.md](Server/README.md) to deploy it on a VPS or home server.
 - Joining a group that already has notes replaces what's on that Mac.
-- The app polls every 5 seconds and pushes 1.5s after a change; last write
-  wins. Changes that came from the server are not echoed back.
+- The app polls every 5 seconds (a tiny version check) and pushes 1.5s after a
+  change; last write wins, and changes that came from the server are not echoed
+  back.
 - Verify sync from a terminal with `Prompt --sync-check`.
 
 ## Build from source