refactor: remove dyndnshosts/drophosts legacy config keys, fix DNS event logging

- Remove dyndnshosts legacy list; dyndns is now set per-host in the hosts section
- Remove drophosts config key and load-time deletion loop
- Simplify get_dyndnshosts() to only read per-host dyndns attributes
- Fix dns_update_worker to call eventlog with correct (host, level, msg) signature
- Log INFO/ERROR events per domain on each DNS update instead of one batched message
- Add logger to dns.py (was missing, causing NameError on update failure)
- Update README and tests to reflect removed config keys

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

hbd/server/dns.py

@@ -4,6 +4,9 @@ from __future__ import annotations
 from subprocess import Popen, PIPE, STDOUT
 from typing import Optional
 import asyncio
+import logging
+
+logger = logging.getLogger(__name__)
 
 
 def create_nsupdate_payload(
@@ -123,7 +126,6 @@ async def dns_update_worker(
                 pass
             continue
 
-        m = f"changed address to {addr}"
         for dyndomain in cfg.get("dyndomains", []):
             err = await loop.run_in_executor(
                 None,
@@ -135,28 +137,29 @@ async def dns_update_worker(
                 cfg.get("rndc_key", "/etc/dhcpc/rndc-key"),
             )
             if err:
-                m += f", DNS update failed: {err}"
+                m = f"DNS update failed for {addr} ({dyndomain}): {err}"
                 logger.error("DNS update failed for %s: %s", name, err)
+                if log:
+                    try:
+                        await loop.run_in_executor(None, log, name, "ERROR", m)
+                    except Exception:
+                        pass
             else:
-                m += ", DNS updated."
+                m = f"DNS updated {name}.dy.{dyndomain} → {addr}"
+                if log:
+                    try:
+                        await loop.run_in_executor(None, log, name, "INFO", m)
+                    except Exception:
+                        pass
+
+        if not cfg.get("dyndomains"):
+            logger.warning("DNS update triggered for %s but no dyndomains configured", name)
 
         try:
             dnsq.task_done()
         except Exception:
             pass
 
-        if log:
-            try:
-                await loop.run_in_executor(None, log, name, m)
-            except Exception:
-                pass
-
-    if log:
-        try:
-            await loop.run_in_executor(None, log, None, "dns_update_worker exiting")
-        except Exception:
-            pass
-
 
 def start_dns_worker(
     hbdclass,