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/config.py

@@ -39,10 +39,8 @@ SERVER_DEFAULTS = {
 
     # Host management
     "hosts": {},                # Unified host definitions
-    "dyndnshosts": [],          # Hosts with dynamic DNS (legacy)
-    "drophosts": [],            # Hosts to ignore
     "dyndomains": ["wrede.org"],
-    
+
     # DNS updates
     "nsupdate_bin": "/usr/bin/nsupdate",
     
@@ -249,7 +247,7 @@ def get_watchhosts(config):
     """Extract watched hostnames from config (hosts with watch: true).
 
     Returns:
-        List of hostnames to watch
+#        List of hostnames to watch
     """
     watchhosts = []
     hosts_config = config.get("hosts", {})
@@ -261,31 +259,14 @@ def get_watchhosts(config):
 
 
 def get_dyndnshosts(config):
-    """Extract dyndnshosts from config, supporting both new and legacy formats.
-    
-    Args:
-        config: Configuration dictionary
-        
-    Returns:
-        List of hostnames with dynamic DNS
-    """
-    dyndnshosts = []
-    
-    # New format: hosts section with dyndns attribute
-    if "hosts" in config:
-        hosts_config = config["hosts"]
-        if isinstance(hosts_config, dict):
-            for host_name, host_attrs in hosts_config.items():
-                if isinstance(host_attrs, dict) and host_attrs.get("dyndns", False):
-                    dyndnshosts.append(host_name)
-    
-    # Legacy format: dyndnshosts list/set
-    if "dyndnshosts" in config:
-        legacy_dyndnshosts = config.get("dyndnshosts", [])
-        if isinstance(legacy_dyndnshosts, (list, set)):
-            dyndnshosts.extend(legacy_dyndnshosts)
-    
-    return list(set(dyndnshosts))  # Remove duplicates
+    """Return hostnames that have a dyndns setting in the hosts section."""
+    hosts_config = config.get("hosts", {})
+    if not isinstance(hosts_config, dict):
+        return []
+    return [
+        name for name, attrs in hosts_config.items()
+        if isinstance(attrs, dict) and attrs.get("dyndns")
+    ]
 
 
 def get_host_config(config, hostname):