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/deploy-bitcoin-knots.sh

@@ -51,7 +51,7 @@ sudo podman run -d \
   docker.io/bitcoinknots/bitcoin:latest \
   -server=1 \
   -txindex=1 \
-  -rpcallowip=0.0.0.0/0 \
+  -rpcallowip=127.0.0.1/32 -rpcallowip=10.88.0.0/16 \
   -rpcbind=0.0.0.0:8332 \
   -rpcuser=archipelago \
   -rpcpassword=$BITCOIN_RPC_PASS \