Trust the RFC 1918 ranges as proxies when TRUSTED_PROXIES is unset (closes #333)

Unset or empty, TRUSTED_PROXIES now defaults to 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16, so a reverse proxy reaching webhooker from one of those ranges gets per-client rate-limit buckets with nothing set. A set value replaces the default entirely; an unparseable one still fails startup. The startup warning for an empty list is gone.

The README gives the default, one rule (if any client can reach webhooker, or the proxy in front of it, from an RFC 1918 source address, set the list to the proxy's address alone), and where to find that address: the remoteIP field of the http request log line. Loopback is not in the default.

Model: opus-5-5

internal/server/routes.go

@@ -154,11 +154,12 @@ func (s *Server) setupPageRoutes() {
 		r.Use(s.mw.NoCache())
 
 		// The login POST carries no pre-emptive rate limiter. Behind
-		// the reverse proxy production requires, with TRUSTED_PROXIES
-		// unset, every client shares one bucket, so a limiter spent
-		// on arrival lets any stranger deny the operator the only
-		// administrative path. The handler verifies credentials first
-		// and charges only failures; see Handlers.authenticateUser.
+		// the reverse proxy production requires, when TRUSTED_PROXIES
+		// does not cover it, every client shares one bucket, so a
+		// limiter spent on arrival lets any stranger deny the operator
+		// the only administrative path. The handler verifies
+		// credentials first and charges only failures; see
+		// Handlers.authenticateUser.
 		r.Get("/login", s.h.HandleLoginPage())
 		r.Post("/login", s.h.HandleLoginSubmit())