feat: implement graceful shutdown on SIGTERM/SIGINT with request draining

Replace http.ListenAndServe with http.Server and signal handling to
support graceful shutdown. On SIGTERM or SIGINT, the server drains
in-flight requests with a 15-second timeout before exiting cleanly.
This prevents dropped connections during Kubernetes pod termination.

Closes leeworks-agents/gitea-mobile#201

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cmd/server/main.go

@@ -1,10 +1,14 @@
 package main
 
 import (
+	"context"
 	"log"
 	"log/slog"
 	"net/http"
 	"os"
+	"os/signal"
+	"syscall"
+	"time"
 
 	"gitea.leeworks.dev/0xwheatyz/gitea-mobile/internal/config"
 	giteaclient "gitea.leeworks.dev/0xwheatyz/gitea-mobile/internal/gitea"
@@ -36,8 +40,35 @@ func main() {
 	handler = middleware.Auth(cfg.SessionSecret, cfg.GiteaToken)(handler)
 	handler = middleware.Logging()(handler)
 
+	srv := &http.Server{
+		Addr:    cfg.ListenAddr,
+		Handler: handler,
+	}
+
+	// Channel to receive shutdown signals.
+	quit := make(chan os.Signal, 1)
+	signal.Notify(quit, syscall.SIGTERM, syscall.SIGINT)
+
+	// Start server in a goroutine.
+	go func() {
 		slog.Info("server starting", "addr", cfg.ListenAddr, "gitea_url", cfg.GiteaURL)
-	if err := http.ListenAndServe(cfg.ListenAddr, handler); err != nil {
+		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
 			log.Fatalf("server error: %v", err)
 		}
+	}()
+
+	// Block until a shutdown signal is received.
+	sig := <-quit
+	slog.Info("shutdown signal received, draining in-flight requests", "signal", sig.String())
+
+	// Give in-flight requests up to 15 seconds to complete.
+	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
+	defer cancel()
+
+	if err := srv.Shutdown(ctx); err != nil {
+		slog.Error("server forced to shutdown", "error", err)
+		os.Exit(1)
+	}
+
+	slog.Info("server stopped gracefully")
 }