fix(nano): drain in-flight logins on shutdown

The installer and the config template tell the operator to run
systemctl restart felis-nano after editing the source list. nano had no
signal handling, so SIGTERM killed it mid-request: a login waiting on an
upstream had its connection reset, and Velocity disconnected that
player with "authentication servers are down". felis api already drains
on shutdown; nano did not.

nano now listens itself, serves until SIGINT or SIGTERM, then shuts the
server down gracefully with a 30-second limit. That outlasts the source
scan of any realistic list, at five seconds per source, and stays well
inside systemd's default 90-second stop timeout.

The new test holds a request inside the handler, cancels the serve
context, and checks that serveNano is still running 200 ms later, that
the held request then gets its answer, and that serveNano returns 0.
Replacing the graceful shutdown with Close fails it.

cmd/felis/nano.go

@@ -29,7 +29,12 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"net"
 	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
 
 	"felis.lolicon.best/internal/api"
 	"felis.lolicon.best/internal/config"
@@ -71,12 +76,39 @@ func cmdNano(args []string, stdout, stderr io.Writer) int {
 		fmt.Fprintf(stderr, "  [%d] %s -> %s\n", i+1, s.Tag, s.URL)
 	}
 
-	srv := newAPIServer(*listen, nanoHandler(cfg.AuthSources, stderr))
-	if err := srv.ListenAndServe(); err != nil {
+	ln, err := net.Listen("tcp", *listen)
+	if err != nil {
 		fmt.Fprintln(stderr, "felis nano:", err)
 		return 1
 	}
-	return 0
+	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	defer stop()
+	return serveNano(ctx, newAPIServer(*listen, nanoHandler(cfg.AuthSources, stderr)), ln, stderr)
+}
+
+// nanoDrainTimeout outlasts the source scan of any realistic list (each source is given
+// five seconds) and stays well inside systemd's default 90-second stop timeout.
+const nanoDrainTimeout = 30 * time.Second
+
+// serveNano serves until ctx ends, then drains. A restart, the documented way to pick up a
+// config edit, sends SIGTERM; without the drain a login already waiting on an upstream has
+// its connection reset, and Velocity tells that player the auth servers are down.
+func serveNano(ctx context.Context, srv *http.Server, ln net.Listener, stderr io.Writer) int {
+	errc := make(chan error, 1)
+	go func() { errc <- srv.Serve(ln) }()
+	select {
+	case err := <-errc:
+		fmt.Fprintln(stderr, "felis nano:", err)
+		return 1
+	case <-ctx.Done():
+		shutdownCtx, cancel := context.WithTimeout(context.Background(), nanoDrainTimeout)
+		defer cancel()
+		if err := srv.Shutdown(shutdownCtx); err != nil {
+			fmt.Fprintln(stderr, "felis nano: shutdown:", err)
+			return 1
+		}
+		return 0
+	}
 }
 
 // nanoHandler is what felis nano serves: the shared hasJoined handler, Mojang first, behind