Serve a master key's devices from several servers

A master key now has a set of servers (the panel's own and/or nodes). A
device's single WireGuard key becomes a peer on every server of its key,
each with its own IP and PSK, and config.servers[] lists them all.

- ui.db: master_servers (the key's set, never empty), device_peers (which
  key is on which server, with the ip/psk that server issued),
  peer_tombstones (peers to delete) and server_profiles (last known
  profile of each server).
- Nothing is pushed blindly: reconcile() brings one server to the desired
  state - deletes tombstones, adds peers, rekeys with PUT, fills ip/psk,
  rotates a pending PSK - one pass per server at a time. It runs right
  after every action, when a node connects and once a minute, so a node
  that was offline catches up on rekeys, PSK changes and removals.
- config is built from ui.db, so an offline node stays in it and rev does
  not flap; a server is listed once its peer carries the device's current
  key. An empty config is never sent (502 instead); register and rekey
  roll back if no server accepted the key.
- Existing keys and devices migrate on start. With one server the config
  and its rev are byte-identical to 0.3.1 (checked by running both
  versions on the same ui.db), so existing clients notice nothing.
- Panel: server checkboxes in the key settings instead of the placeholder
  (the last one cannot be removed), "на N из M серверов" on devices that
  have not reached every server yet, device stats summed over servers.
- SUB_HOST: the panel's public address for sen:// links and endpoints;
  install.sh writes it. Without it a panel with no VPN of its own used
  the address it was opened at.
- A node referenced by a key's server set cannot be deleted.

install.sh

@@ -1363,6 +1363,7 @@ SUB_TLS=${SUB_TLS}
 SUB_SIGN_KEY_FILE=${SUB_SIGN_PRIV}
 SUB_TLS_KEY_FILE=${SUB_TLS_KEY}
 SUB_TLS_CERT_FILE=${SUB_TLS_CRT}
+SUB_HOST=${SERVER_IP}
 
 # ── приём нод (несколько серверов) ────────────────────────────────────────
 NODE_PORT=${NODE_PORT}