Restrict /s/* to GET and HEAD (closes #169)

The static file server was attached with Mount, which registers every
method, so POST, PUT and DELETE on an asset were answered 200 with the
file. It is now registered for GET and HEAD only, inside a /s group
whose method-not-allowed handler answers 405 with Allow: GET, HEAD
(chi's default 405 sends no Allow header).

TestStaticServesEveryMethod is inverted and renamed
TestStaticServesOnlyGetAndHead, and the README route table row for
/s/* now says the same.

Model: opus-5-5

internal/server/routes.go

@@ -92,11 +92,25 @@ func (s *Server) setupGlobalMiddleware() {
 func (s *Server) setupRoutes() {
 	s.router.Get("/", s.h.HandleIndex())
 
-	s.router.Mount(
-		"/s",
-		http.StripPrefix("/s", http.FileServer(http.FS(static.Static))),
+	// Static assets answer GET and HEAD only. chi's default 405
+	// carries no Allow header, so this group supplies its own.
+	staticFiles := http.StripPrefix(
+		"/s", http.FileServer(http.FS(static.Static)),
 	)
 
+	s.router.Route("/s", func(r chi.Router) {
+		r.MethodNotAllowed(func(w http.ResponseWriter, _ *http.Request) {
+			w.Header().Set("Allow", "GET, HEAD")
+			http.Error(
+				w,
+				"Method Not Allowed",
+				http.StatusMethodNotAllowed,
+			)
+		})
+		r.Method(http.MethodGet, "/*", staticFiles)
+		r.Method(http.MethodHead, "/*", staticFiles)
+	})
+
 	s.router.Route("/api/v1", func(_ chi.Router) {
 		// API routes will be added here.
 	})

internal/server/routes_test.go

@@ -396,13 +396,12 @@ func (e *testEnv) storedHash(t *testing.T, username string) string {
 
 // --- /s static group ---
 
-// TestStaticServesEveryMethod pins what the static mount actually
-// answers. chi's Mount registers the handler for all methods and
-// http.FileServer only special-cases HEAD (by suppressing the body),
-// so a POST or a DELETE to an asset is served the file rather than
-// refused. The README documents this; the test is what keeps the two
-// from drifting.
-func TestStaticServesEveryMethod(t *testing.T) {
+// TestStaticServesOnlyGetAndHead pins the methods the static group
+// answers: GET and HEAD are served the asset, and any other method
+// is refused with 405 and an Allow header naming those two. The
+// README documents this; the test is what keeps the two from
+// drifting.
+func TestStaticServesOnlyGetAndHead(t *testing.T) {
 	t.Parallel()
 
 	env := newTestEnv(t)
@@ -428,18 +427,27 @@ func TestStaticServesEveryMethod(t *testing.T) {
 			w := httptest.NewRecorder()
 			env.router.ServeHTTP(w, req)
 
-			assert.Equal(t, http.StatusOK, w.Code,
-				"static mount answers every method")
-
-			if method == http.MethodHead {
+			switch method {
+			case http.MethodGet:
+				assert.Equal(t, http.StatusOK, w.Code)
+				assert.Equal(t, body, w.Body.Bytes(),
+					"the asset itself is returned")
+			case http.MethodHead:
+				assert.Equal(t, http.StatusOK, w.Code)
 				assert.Empty(t, w.Body.Bytes(),
 					"HEAD must not carry a body")
-
-				return
+			default:
+				assert.Equal(
+					t, http.StatusMethodNotAllowed, w.Code,
+				)
+				assert.Equal(
+					t, "GET, HEAD", w.Header().Get("Allow"),
+				)
+				assert.NotContains(
+					t, w.Body.String(), string(body),
+					"a refused method must not get the asset",
+				)
 			}
-
-			assert.Equal(t, body, w.Body.Bytes(),
-				"the asset itself is returned")
 		})
 	}
 }