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>

core/archipelago/src/api/rpc/package.rs

@@ -314,7 +314,8 @@ prune=550\n\
 rpcuser=archipelago\n\
 rpcpassword={}\n\
 rpcbind=0.0.0.0\n\
-rpcallowip=0.0.0.0/0\n\
+rpcallowip=127.0.0.1/32\n\
+rpcallowip=10.88.0.0/16\n\
 rpcport=8332\n\
 listen=1\n\
 printtoconsole=1\n", rpc_pass);