fix: server starts even without config (REQUIRE_SETUP mode)

main.go no longer fatals on ErrRequireSetup — it passes cfg=nil to NewServer.
NewServer returns REQUIRE_SETUP on every GraphQL call when cfg is nil, and
also adds CORS headers and serves the Vue SPA from /srv/archivum/ui.

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

backend/cmd/server/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"log"
 	"net/http"
 	"os"
@@ -11,14 +12,20 @@ import (
 
 func main() {
 	cfg, err := config.Load(configPath())
-	if err != nil {
+	if err != nil && !errors.Is(err, config.ErrRequireSetup) {
 		log.Fatalf("failed to load config: %v", err)
 	}
 
+	// 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)
 
-	log.Printf("Archivum listening on %s", cfg.ListenAddr)
-	if err := http.ListenAndServe(cfg.ListenAddr, srv); err != nil {
+	log.Printf("Archivum listening on %s", addr)
+	if err := http.ListenAndServe(addr, srv); err != nil {
 		log.Fatal(err)
 	}
 }