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>

README.md

@@ -8,7 +8,7 @@ A lightweight daemon that listens for UDP heartbeat messages and acts on them: k
 
 - Receive and parse heartbeat datagrams (text or zlib-compressed) ✅
 - Maintain host state and detect up/down transitions ✅
-- Queue DNS updates via `nsupdate` and run them in a background thread ✅
+- Queue DNS updates via `nsupdate` and run them in an asyncio background task ✅
 - WebSocket API for live updates (hosts & messages) ✅
 - Notification pipeline (email, Pushover, Mattermost, Signal) ✅
 - **User management & access control** ✅
@@ -398,6 +398,7 @@ hosts:
     owner: alice
     managers: [bob]
     monitors: [carol]
+    dyndns: true          # update DNS record when IP changes
 ```
 
 ```bash
@@ -645,7 +646,7 @@ Set breakpoints in modules such as `hbd/server/udp.py`, `hbd/server/dns.py`, or
 - `logfile`: path to log file
 - `pushsrv`: push service (`pushover`|`mattermost`|`all`)
 - `interval` / `grace`: heartbeat timing configuration
-- `dyndomains`: list of dyndomains to update via `nsupdate`
+- `dyndomains`: list of DNS domains to update via `nsupdate` for hosts with `dyndns` set
 - `nsupdate_bin`: path to nsupdate binary
 - `ws_port`: port for plain WebSocket connections (default: 50005)
 - `wss_port`: port for secure WebSocket (WSS) connections (default: none).
@@ -666,6 +667,9 @@ dyndomains:
   - example.com
 nsupdate_bin: /usr/bin/nsupdate
 pushsrv: pushover
+hosts:
+  myhost:
+    dyndns: true          # update DNS when this host's IP changes
 ```
 
 > Tip: `SERVER_DEFAULTS` in `hbd/server/config.py` contains the canonical defaults and accepted configuration keys.
@@ -769,10 +773,3 @@ Contributions welcome! Please:
 This repository is licensed under the MIT license. See `LICENSE` for details.
 
 ---
-
-If you'd like, I can also:
-
-- add a **GitHub Actions** workflow that runs tests and lint on push/PR 🔁
-- add a `CONTRIBUTING.md` template for PRs and code style 💬
-
-Which one should I do next? ✨