Pin the body cap's order in every page route group (closes #93)

Follow-ups from an August review of the body cap, each checked against the current tree. One route test now requires an oversized POST, with no session and no CSRF token, to be refused with 413 before CSRF runs, in every page route group with a POST route and in /settings/, so moving a group's body cap after CSRF fails it. The three test router helpers build the server through New with a lifecycle that is never started, so no field is set by hand. The middleware test comment names runMaxBodySize, and the MaxBodySize doc comment says methods other than POST, PUT and PATCH pass uncapped on purpose. The README item was already settled.

Model: opus-5-5

internal/middleware/middleware_test.go

@@ -730,10 +730,8 @@ func TestNoCache_SetsHeaders(t *testing.T) {
 
 const testBodyLimit int64 = 64
 
-// maxBodySizeHandler wraps a sentinel handler in MaxBodySize with
-// testBodyLimit. The sentinel records whether it ran and how much of
-// the body it managed to read, so tests can distinguish "never
-// reached" from "reached but truncated".
+// maxBodySizeResult is what runMaxBodySize's sentinel handler saw,
+// together with the response.
 type maxBodySizeResult struct {
 	called   bool
 	read     int
@@ -741,6 +739,10 @@ type maxBodySizeResult struct {
 	response *httptest.ResponseRecorder
 }
 
+// runMaxBodySize wraps a sentinel handler in MaxBodySize with
+// testBodyLimit and serves req through it. The sentinel records
+// whether it ran and how much of the body it managed to read, so
+// tests can distinguish "never reached" from "reached but truncated".
 func runMaxBodySize(
 	t *testing.T,
 	req *http.Request,