fix+feat: real setup persistence, request logging, improved SPA serving

- server.go now accepts configPath so setup mutation writes config.json to disk
- setup mutation parses GraphQL variables and persists config — no more setup loop
- request logger middleware logs every HTTP request with method/path/status/duration
- GraphQL handler logs operation name and setup/status decisions
- spaHandler simplified: serves real files directly, falls back to index.html
  only for paths that don't exist (fixes potential asset-serving issues)
- health endpoint returns JSON and 503 in setup mode
- main.go logs config path, storage path and db path on startup

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/cmd/server/main.go

@@ -11,18 +11,26 @@ import (
 )
 
 func main() {
-	cfg, err := config.Load(configPath())
-	if err != nil && !errors.Is(err, config.ErrRequireSetup) {
-		log.Fatalf("failed to load config: %v", err)
+	cfgPath := configPath()
+	log.Printf("config path: %s", cfgPath)
+
+	cfg, err := config.Load(cfgPath)
+	if err != nil {
+		if errors.Is(err, config.ErrRequireSetup) {
+			log.Printf("no config found — starting in setup mode (wizard at http://localhost:4000/)")
+		} else {
+			log.Fatalf("failed to load config: %v", err)
+		}
+	} else {
+		log.Printf("config loaded — storage: %s  db: %s", cfg.StoragePath, cfg.DBPath)
 	}
 
-	// cfg may be nil when REQUIRE_SETUP — NewServer handles that gracefully.
 	addr := ":4000"
 	if cfg != nil && cfg.ListenAddr != "" {
 		addr = cfg.ListenAddr
 	}
 
-	srv := graph.NewServer(cfg)
+	srv := graph.NewServer(cfg, cfgPath)
 
 	log.Printf("Archivum listening on %s", addr)
 	if err := http.ListenAndServe(addr, srv); err != nil {