init: added the base files by ai and debugged by hand

it's great but currently it has a basic error which is go nil pointer dereference

WIP

main.go

@@ -0,0 +1,75 @@
+// main.go — auth-gate entry point
+//
+// This file wires together configuration loading, the HTTP server, and the
+// background processes (permanent-whitelist file watcher, temporary-whitelist
+// cleanup).
+//
+// Why this structure?
+//
+// - Main keeps the wiring thin. Each component (config, auth, cleanup, watcher)
+//   owns its own logic so we can test and reason about them independently.
+// - We don't embed the HTTP server logic inside main; instead main calls
+//   createAuthHandler(config) and server.Serve(). This makes the handler
+//   functionally pure (aside from config reads) and easier to mock.
+// - The background goroutines are started explicitly so we can stop them on
+//   SIGTERM without leaking resources.
+
+package main
+
+import (
+	"context"
+	"log"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+)
+
+func main() {
+	cfg := loadConfig()
+
+	// Load the permanent whitelist from disk once at startup.
+	// We reload it periodically via watcher (see watcher.go).
+	loadPermanentWhitelist(cfg.PermanentWhitelistFile)
+
+	// Start the background watcher that reloads the permanent whitelist file.
+	// It only reloads when the file actually changes (mtime comparison), so
+	// frequent touch-operations don't cause unnecessary restarts.
+	w := newPermanentWhitelistWatcher(cfg.PermanentWhitelistFile, cfg.WatchInterval, cfg.PermanentWhitelist)
+	w.start()
+
+	// Start the cleanup goroutine that expires temporary whitelisted IPs
+	// whose TTL has elapsed. It runs every cfg.CleanupInterval.
+	go cleanupLoop(cfg.TemporaryWhitelist, cfg.CleanupInterval)
+
+	// Build the HTTP server.
+	addr := ":" + cfg.Port
+	server := newAuthServer(cfg, addr)
+
+	// Start serving in a goroutine so we can do graceful shutdown.
+	go func() {
+		log.Printf("auth-gate listening on %s", addr)
+		if err := server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+			log.Fatalf("server error: %v", err)
+		}
+	}()
+
+	// Graceful shutdown on SIGTERM / SIGINT.
+	//
+	// We wait for a signal, then call server.Shutdown(ctx) which:
+	//   1. Stops accepting new connections
+	//   2. Closes idle connections
+	//   3. Waits for in-flight requests to complete (up to 30s)
+	//
+	// This is important for Docker deployments: without it the container
+	// would be killed mid-request and clients would see connection errors.
+	quit := make(chan os.Signal, 1)
+	signal.Notify(quit, syscall.SIGTERM, syscall.SIGINT)
+	<-quit
+
+	log.Println("shutting down...")
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+	server.Shutdown(ctx)
+}