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>

tests/test_handle_datagram.py

@@ -20,7 +20,7 @@ def test_handle_cmd_sends_command():
     import hbdclass
 
     ctx = {
-        "config": {"watchhosts": [], "dyndnshosts": []},
+        "config": {"watchhosts": []},
         "hbdclass": hbdclass,
         "log": dummy_noop,
         "email": dummy_noop,