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.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.