Document and test that unrouted methods get 405 without Allow on /s/*

A method chi does not route, such as PROPFIND, is refused by the
top-level router before it reaches the /s group, so it gets 405
without an Allow header. The README row and the test's doc comment
now say so, and TestStaticServesOnlyGetAndHead checks PROPFIND.

Model: opus-5-5

README.md

@@ -2721,7 +2721,7 @@ abuse limit later; they are tracked as future work.
 | ------ | --------------------------- | ----------- |
 | `GET`  | `/`                         | Root redirect, 303 (authenticated → `/sources`, unauthenticated → `/pages/login`) |
 | `GET`  | `/.well-known/healthcheck`  | Health check (JSON: `status`, `now`, `uptimeSeconds`, `uptimeHuman`, `version`, `appname`, `maintenanceMode`) |
-| `GET`, `HEAD` | `/s/*`              | Static file serving (embedded CSS, JS). `GET` and `HEAD` only — every other method is answered `405 Method Not Allowed` with `Allow: GET, HEAD`. Pinned by `TestStaticServesOnlyGetAndHead` |
+| `GET`, `HEAD` | `/s/*`              | Static file serving (embedded CSS, JS). `GET` and `HEAD` only — `POST`, `PUT`, `PATCH`, `DELETE`, `OPTIONS`, `TRACE` and `CONNECT` are answered `405 Method Not Allowed` with `Allow: GET, HEAD`. Any other method (such as `PROPFIND`) is refused by chi before it reaches this route, and gets `405` without an `Allow` header. Pinned by `TestStaticServesOnlyGetAndHead` |
 | `POST` | `/webhook/{uuid}`           | Webhook receiver endpoint. `POST` only — every other method is answered `405 Method Not Allowed` with `Allow: POST`. Rate limited (see [Rate Limiting](#rate-limiting)) |
 
 #### Authentication Endpoints

internal/server/routes_test.go

@@ -397,9 +397,12 @@ func (e *testEnv) storedHash(t *testing.T, username string) string {
 // --- /s static group ---
 
 // TestStaticServesOnlyGetAndHead pins the methods the static group
-// answers: GET and HEAD are served the asset, and any other method
+// answers: GET and HEAD are served the asset, and the other methods
-// is refused with 405 and an Allow header naming those two. The
+// chi routes (POST, PUT, DELETE and the rest) are refused with 405
-// README documents this; the test is what keeps the two from
+// and an Allow header naming those two. A method chi does not route,
+// such as PROPFIND, is refused with 405 by the top-level router
+// before it reaches the static group, so it gets no Allow header.
+// The README documents this; the test is what keeps the two from
 // drifting.
 func TestStaticServesOnlyGetAndHead(t *testing.T) {
 	t.Parallel()
@@ -416,6 +419,7 @@ func TestStaticServesOnlyGetAndHead(t *testing.T) {
 		http.MethodPost,
 		http.MethodPut,
 		http.MethodDelete,
+		"PROPFIND",
 	} {
 		t.Run(method, func(t *testing.T) {
 			t.Parallel()
@@ -436,6 +440,17 @@ func TestStaticServesOnlyGetAndHead(t *testing.T) {
 				assert.Equal(t, http.StatusOK, w.Code)
 				assert.Empty(t, w.Body.Bytes(),
 					"HEAD must not carry a body")
+			case "PROPFIND":
+				assert.Equal(
+					t, http.StatusMethodNotAllowed, w.Code,
+				)
+				assert.Empty(t, w.Header().Get("Allow"),
+					"chi refuses a method it does not route "+
+						"before the static group runs")
+				assert.NotContains(
+					t, w.Body.String(), string(body),
+					"a refused method must not get the asset",
+				)
 			default:
 				assert.Equal(
 					t, http.StatusMethodNotAllowed, w.Code,