fix: implement 22 security pentest remediation fixes

Server-side session management with SHA-256 hashed tokens and HttpOnly
cookies. Auth middleware gating all RPC/WS/proxy routes with method
allowlist. Login rate limiting (5/60s per IP). CORS restricted to
config origin. Docker registry allowlist. App ID and path validation.
P2P message sanitization (HTML + log injection). Onion address and
known-peer validation. Nginx security headers (CSP, X-Frame-Options,
etc.) and AIUI proxy auth. Systemd hardening (non-root, NoNewPrivileges,
ProtectSystem).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/archipelago/src/main.rs

@@ -18,6 +18,7 @@ mod node_message;
 mod nostr_discovery;
 mod peers;
 mod server;
+mod session;
 mod state;
 
 use auth::AuthManager;