Enforce the body size limit before CSRF parses the form (closes #90)

CSRF ran before MaxBodySize, so the CSRF middleware parsed the form body
before any cap applied and an oversized request was read in full before
being rejected. MaxBodySize is now the first middleware in all four route
groups that parse forms, ahead of CSRF and RequireAuth.

An oversize request therefore gets 413 without the handler running and
without state changing, including the password-change route.

Note the ordering trade: an unauthenticated client now receives 413 rather
than an auth redirect on /user/{username}/password.

internal/handlers/auth.go

@@ -29,10 +29,8 @@ func (h *Handlers) HandleLoginPage() http.HandlerFunc {
 // HandleLoginSubmit handles the login form submission (POST)
 func (h *Handlers) HandleLoginSubmit() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		// Limit request body to prevent memory exhaustion
-		r.Body = http.MaxBytesReader(w, r.Body, 1<<maxBodyShift)
-
-		// Parse form data
+		// The body size cap is enforced by the MaxBodySize
+		// middleware, which runs before CSRF parses the form.
 		err := r.ParseForm()
 		if err != nil {
 			h.log.Error("failed to parse form", "error", err)