hbc/server: request InfoPlugin refresh when host has no plugin data; update docs

- Server sets request_update=1 in ACK when host.plugin_data is empty
- hbc: AsyncConnection.request_info_event; handle_ack sets it on request_update
- hbc: _info_plugin_refresh_loop clears InfoPlugin caches and resends on demand
- hbc_mini: same via _request_info event and _info_refresh_loop
- docs/USERS.md: document client-declared owner config key
- docs/PLUGIN_DEVELOPMENT.md: document server-initiated InfoPlugin refresh

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

hbd/server/udp.py

@@ -350,8 +350,10 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
 
     if msg.get("ID") == "HTB":
         host.doesack = msg.get("acks", -1)
-        # send ACK back
+        # send ACK back; ask client to resend plugin info when we have none yet
         rmsg = {"time": time.time()}
+        if not host.plugin_data:
+            rmsg["request_update"] = 1
         opkt = dicttos("ACK", rmsg)
         try:
             transport.sendto(opkt, addr)