server: drop in-process TLS — HTTP-only behind reverse proxy

Self-hosted deployments already terminate TLS at Caddy/Traefik/nginx;
making the server do TLS too means double cert config, dual ACME
plumbing, and an untested code path. Drop RM_TLS_CERT/RM_TLS_KEY,
remove TLSEnabled() and the ListenAndServeTLS branch.

Replace the cookie's "Secure if TLS-in-process" check with a new
RM_COOKIE_SECURE flag (default true). Local HTTP-only testing sets
RM_COOKIE_SECURE=false; production is always behind a TLS proxy and
the cookie stays Secure.

Default port :8443 → :8080. docker-compose binds 127.0.0.1 only and
populates RM_TRUSTED_PROXY. spec.md §4.1/§10.1 rewritten with a
Caddyfile snippet and a hard "do not expose RM_LISTEN publicly"
warning. enrollResponse keeps cert_pin_sha256 in the shape but the
server can't introspect a cert it doesn't terminate — operator
pastes the proxy's hash into -cert-pin at install time.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/server/main.go

@@ -47,7 +47,7 @@ func run() error {
 		return fmt.Errorf("config: %w", err)
 	}
 	slog.Info("config resolved", "listen", cfg.Listen, "data_dir", cfg.DataDir,
-		"tls", cfg.TLSEnabled(), "trusted_proxies", cfg.TrustedProxies)
+		"cookie_secure", cfg.CookieSecure, "trusted_proxies", cfg.TrustedProxies)
 
 	if err := os.MkdirAll(cfg.DataDir, 0o700); err != nil {
 		return fmt.Errorf("ensure data dir: %w", err)