feat: Phase 2 — systemd sandboxing, Bitcoin RPC localhost binding, Tailscale deprivilege

- Service runs as unprivileged `archipelago` user instead of root
- Added systemd sandboxing: ProtectSystem=strict, NoNewPrivileges, PrivateTmp,
  MemoryDenyWriteExecute, RestrictNamespaces, SystemCallFilter
- Bitcoin RPC rpcallowip restricted to localhost + Podman subnet (10.88.0.0/16)
- Tailscale container: removed --privileged, uses cap-drop ALL + cap-add NET_ADMIN/NET_RAW

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/container-doctor.sh

@@ -261,7 +261,8 @@ prune=550
 rpcuser=archipelago
 rpcpassword=$BTC_RPC_PASS
 rpcbind=0.0.0.0
-rpcallowip=0.0.0.0/0
+rpcallowip=127.0.0.1/32
+rpcallowip=10.88.0.0/16
 rpcport=8332
 listen=1
 printtoconsole=1