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

Server/prompt-sync.service

@@ -0,0 +1,22 @@
+[Unit]
+Description=Prompt sync server
+Documentation=https://gitea.tzgyn.dev/TZGyn/prompt
+After=network-online.target postgresql.service
+Wants=network-online.target
+
+[Service]
+# The binary is built on a Mac with:
+#   GOOS=linux GOARCH=arm64 go build -o prompt-sync .
+ExecStart=/usr/local/bin/prompt-sync
+Environment=PROMPT_SYNC_ADDR=:8080
+Environment=PROMPT_SYNC_DATABASE_URL=postgres://prompt:CHANGE_ME@localhost:5432/prompt_sync?sslmode=disable
+Restart=on-failure
+RestartSec=2
+# The server only needs to talk to Postgres and bind :8080.
+NoNewPrivileges=true
+PrivateTmp=true
+ProtectSystem=strict
+ProtectHome=true
+
+[Install]
+WantedBy=multi-user.target