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

chi runs Use middleware in registration order, and every form route
group registered CSRF() before MaxBodySize(). gorilla/csrf calls
r.PostFormValue, so the form was parsed under net/http's 10 MB default
and the intended 1 MB cap never applied to form fields. The
/user/{username} group, which carries POST /password, had no
MaxBodySize registration at all.

- Register MaxBodySize ahead of CSRF in /pages, /sources, and
  /source/{sourceID}, and add it to /user/{username}.
- Reject a declared-oversize body up front with 413. Reordering alone
  cannot produce one: http.MaxBytesReader surfaces its error on Read,
  so the form parse fails and gorilla/csrf answers 403 "no token" for
  what is really an oversized body. MaxBytesReader is still installed
  afterwards so chunked or length-lying clients stay hard-capped.
- Drop the handler-local MaxBytesReader calls in auth.go, profile.go,
  and source_management.go now that the middleware is the single
  enforcement point. maxBodyShift stays; webhook.go still uses it.

The /webhook/{uuid} receiver is untouched: it bounds itself with
io.LimitReader in readWebhookBody and is neither CSRF-protected nor
form-parsed.

Tests cover the middleware in isolation (declared oversize is rejected
without reaching a sentinel handler; at-limit and under-limit bodies
pass through intact; GET is unaffected; an undeclared oversize body is
truncated at the cap) and the real router built by SetupRoutes, so the
registration order itself is guarded: an oversized POST to
/pages/login returns 413 with no gorilla/csrf cookie issued, an
oversized POST /password with a valid session and CSRF token returns
413 and leaves the stored hash unchanged, and under-limit requests
still complete through the normal CSRF path.

internal/server/routes.go

@@ -90,9 +90,11 @@ func (s *Server) setupRoutes() {
 
 func (s *Server) setupPageRoutes() {
 	s.router.Route("/pages", func(r chi.Router) {
+		// MaxBodySize must precede CSRF: gorilla/csrf parses the
+		// form, so the cap has to be installed before it runs.
+		r.Use(s.mw.MaxBodySize(maxFormBodySize))
 		r.Use(s.mw.CSRF())
 		r.Use(s.mw.NoCache())
-		r.Use(s.mw.MaxBodySize(maxFormBodySize))
 
 		r.Group(func(r chi.Router) {
 			r.Use(s.mw.LoginRateLimit())
@@ -106,6 +108,9 @@ func (s *Server) setupPageRoutes() {
 
 func (s *Server) setupUserRoutes() {
 	s.router.Route("/user/{username}", func(r chi.Router) {
+		// MaxBodySize must precede CSRF: gorilla/csrf parses the
+		// form, so the cap has to be installed before it runs.
+		r.Use(s.mw.MaxBodySize(maxFormBodySize))
 		r.Use(s.mw.CSRF())
 		r.Use(s.mw.NoCache())
 		r.Use(s.mw.RequireAuth())
@@ -118,20 +123,24 @@ func (s *Server) setupUserRoutes() {
 
 func (s *Server) setupSourceRoutes() {
 	s.router.Route("/sources", func(r chi.Router) {
+		// MaxBodySize must precede CSRF: gorilla/csrf parses the
+		// form, so the cap has to be installed before it runs.
+		r.Use(s.mw.MaxBodySize(maxFormBodySize))
 		r.Use(s.mw.CSRF())
 		r.Use(s.mw.NoCache())
 		r.Use(s.mw.RequireAuth())
-		r.Use(s.mw.MaxBodySize(maxFormBodySize))
 		r.Get("/", s.h.HandleSourceList())
 		r.Get("/new", s.h.HandleSourceCreate())
 		r.Post("/new", s.h.HandleSourceCreateSubmit())
 	})
 
 	s.router.Route("/source/{sourceID}", func(r chi.Router) {
+		// MaxBodySize must precede CSRF: gorilla/csrf parses the
+		// form, so the cap has to be installed before it runs.
+		r.Use(s.mw.MaxBodySize(maxFormBodySize))
 		r.Use(s.mw.CSRF())
 		r.Use(s.mw.NoCache())
 		r.Use(s.mw.RequireAuth())
-		r.Use(s.mw.MaxBodySize(maxFormBodySize))
 		r.Get("/", s.h.HandleSourceDetail())
 		r.Get("/edit", s.h.HandleSourceEdit())
 		r.Post("/edit", s.h.HandleSourceEditSubmit())