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/main.go

@@ -6,6 +6,7 @@
 //
 //	POST /api/codes           -> {"code":"1234-5678"}        (create group)
 //	GET  /api/sync/{code}     -> {"version":N,"data":{...}}  (404 if unknown)
+//	GET  /api/sync/{code}/version -> {"version":N}           (cheap poll)
 //	PUT  /api/sync/{code}     -> {"version":N+1}             (upsert payload)
 //	GET  /healthz             -> 200
 //
@@ -16,6 +17,7 @@
 package main
 
 import (
+	"context"
 	"crypto/rand"
 	"database/sql"
 	"encoding/json"
@@ -23,7 +25,9 @@ import (
 	"log"
 	"net/http"
 	"os"
+	"os/signal"
 	"regexp"
+	"syscall"
 	"time"
 
 	_ "github.com/lib/pq"
@@ -69,13 +73,55 @@ func main() {
 	mux := http.NewServeMux()
 	mux.HandleFunc("POST /api/codes", s.createCode)
 	mux.HandleFunc("GET /api/sync/{code}", s.getSync)
+	mux.HandleFunc("GET /api/sync/{code}/version", s.getVersion)
 	mux.HandleFunc("PUT /api/sync/{code}", s.putSync)
 	mux.HandleFunc("GET /healthz", func(w http.ResponseWriter, _ *http.Request) {
 		w.WriteHeader(http.StatusOK)
 	})
 
+	server := &http.Server{
+		Addr:              addr,
+		Handler:           withLogging(mux),
+		ReadHeaderTimeout: 10 * time.Second,
+	}
+
+	// Shut down cleanly so a redeploy (systemd restart, container stop) never
+	// cuts off an in-flight push and leaves a client thinking it failed.
+	stop := make(chan os.Signal, 1)
+	signal.Notify(stop, os.Interrupt, syscall.SIGTERM)
+	go func() {
+		<-stop
+		log.Println("shutting down")
+		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		defer cancel()
+		_ = server.Shutdown(ctx)
+	}()
+
 	log.Printf("prompt-sync listening on %s", addr)
-	log.Fatal(http.ListenAndServe(addr, mux))
+	if err := server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+		log.Fatalf("serve: %v", err)
+	}
+}
+
+// withLogging keeps a one-line trail of method, path, status and duration so a
+// self-hosted deployment can be debugged from journalctl or docker logs.
+func withLogging(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		started := time.Now()
+		recorder := &statusRecorder{ResponseWriter: w, status: http.StatusOK}
+		next.ServeHTTP(recorder, r)
+		log.Printf("%s %s %d %s", r.Method, r.URL.Path, recorder.status, time.Since(started).Round(time.Millisecond))
+	})
+}
+
+type statusRecorder struct {
+	http.ResponseWriter
+	status int
+}
+
+func (r *statusRecorder) WriteHeader(status int) {
+	r.status = status
+	r.ResponseWriter.WriteHeader(status)
 }
 
 // createCode generates a unique 8-digit code, formats it as 1234-5678, and
@@ -136,6 +182,27 @@ func (s *Server) getSync(w http.ResponseWriter, r *http.Request) {
 		version, updatedAt.UTC().Format(time.RFC3339), data)
 }
 
+// getVersion is the cheap poll: clients ask for the version every few seconds
+// and only download the payload (which can carry base64 images) when it moved.
+func (s *Server) getVersion(w http.ResponseWriter, r *http.Request) {
+	code := r.PathValue("code")
+	if !codePattern.MatchString(code) {
+		writeError(w, http.StatusBadRequest, "code must look like 1234-5678")
+		return
+	}
+	var version int64
+	err := s.db.QueryRow(`SELECT version FROM sync_groups WHERE code = $1`, code).Scan(&version)
+	if err == sql.ErrNoRows {
+		writeError(w, http.StatusNotFound, "unknown code")
+		return
+	}
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "database error")
+		return
+	}
+	writeJSON(w, http.StatusOK, map[string]int64{"version": version})
+}
+
 func (s *Server) putSync(w http.ResponseWriter, r *http.Request) {
 	code := r.PathValue("code")
 	if !codePattern.MatchString(code) {