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/configio.py

@@ -25,7 +25,7 @@ _SERVER_KEYS = [
 ]
 
 # Top-level keys managed by the 'dns' logical section
-_DNS_KEYS = ["nsupdate_bin", "dyndomains", "dyndnshosts", "drophosts"]
+_DNS_KEYS = ["nsupdate_bin", "rndc_key", "dyndomains"]
 
 
 def read_roundtrip(path: str):