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

A route test now posts an oversized body with no session or CSRF
token to each page route group, /settings included, and requires 413
with no CSRF cookie. Before, only the login form pinned the cap ahead
of CSRF; reordering the /settings, /hooks or /hook groups failed
nothing.

The MaxBodySize doc comment says other methods pass uncapped on
purpose, and the middleware test comment names the helper it
describes. The three router helpers in the server tests build the
Server through New, on a lifecycle that is never started, instead of
setting its fields by hand. The README already described the cap's
position correctly.

Model: opus-5-5

internal/middleware/middleware.go

@@ -600,7 +600,10 @@ func bodyLimitedMethod(method string) bool {
 }
 
 // MaxBodySize returns middleware that limits the size of
-// POST/PUT/PATCH request bodies to maxBytes. It must be registered
+// POST/PUT/PATCH request bodies to maxBytes. A request with any other
+// method passes through uncapped, deliberately: no route behind it
+// reads a body on GET, HEAD or DELETE. A handler that starts to needs
+// its method added to bodyLimitedMethod first. It must be registered
 // before any middleware that parses the body — notably CSRF, which
 // calls r.PostFormValue — so that form parsing happens under this
 // cap rather than net/http's 10 MB default.