feat: unify DOMAINS/HOSTNAMES into single TARGETS config

Replace DNSWATCHER_DOMAINS and DNSWATCHER_HOSTNAMES with a single
DNSWATCHER_TARGETS env var. Names are automatically classified as apex
domains or hostnames using the Public Suffix List
(golang.org/x/net/publicsuffix).

- ClassifyDNSName() uses EffectiveTLDPlusOne to determine type
- Public suffixes themselves (e.g. co.uk) are rejected with an error
- Old DOMAINS/HOSTNAMES vars removed entirely (pre-1.0, no compat needed)
- README updated with pre-1.0 warning

Closes #10

README.md

@@ -1,5 +1,7 @@
 # dnswatcher
 
+> ⚠️ Pre-1.0 software. APIs, configuration, and behavior may change without notice.
+
 dnswatcher is a production DNS and infrastructure monitoring daemon written in
 Go.  It watches configured DNS domains and hostnames for changes, monitors TCP
 port availability, tracks TLS certificate expiry, and delivers real-time
@@ -195,8 +197,7 @@ the following precedence (highest to lowest):
 | `PORT`                          | HTTP listen port                           | `8080`      |
 | `DNSWATCHER_DEBUG`              | Enable debug logging                       | `false`     |
 | `DNSWATCHER_DATA_DIR`           | Directory for state file                   | `./data`    |
-| `DNSWATCHER_DOMAINS`            | Comma-separated list of apex domains       | `""`        |
-| `DNSWATCHER_HOSTNAMES`          | Comma-separated list of hostnames          | `""`        |
+| `DNSWATCHER_TARGETS`            | Comma-separated DNS names (auto-classified via PSL) | `""`  |
 | `DNSWATCHER_SLACK_WEBHOOK`      | Slack incoming webhook URL                 | `""`        |
 | `DNSWATCHER_MATTERMOST_WEBHOOK` | Mattermost incoming webhook URL            | `""`        |
 | `DNSWATCHER_NTFY_TOPIC`         | ntfy topic URL                             | `""`        |
@@ -214,8 +215,7 @@ the following precedence (highest to lowest):
 PORT=8080
 DNSWATCHER_DEBUG=false
 DNSWATCHER_DATA_DIR=./data
-DNSWATCHER_DOMAINS=example.com,example.org
-DNSWATCHER_HOSTNAMES=www.example.com,api.example.com,mail.example.org
+DNSWATCHER_TARGETS=example.com,example.org,www.example.com,api.example.com,mail.example.org
 DNSWATCHER_SLACK_WEBHOOK=https://hooks.slack.com/services/T.../B.../xxx
 DNSWATCHER_MATTERMOST_WEBHOOK=https://mattermost.example.com/hooks/xxx
 DNSWATCHER_NTFY_TOPIC=https://ntfy.sh/my-dns-alerts
@@ -352,8 +352,7 @@ docker build -t dnswatcher .
 docker run -d \
   -p 8080:8080 \
   -v dnswatcher-data:/var/lib/dnswatcher \
-  -e DNSWATCHER_DOMAINS=example.com \
-  -e DNSWATCHER_HOSTNAMES=www.example.com \
+  -e DNSWATCHER_TARGETS=example.com,www.example.com \
   -e DNSWATCHER_NTFY_TOPIC=https://ntfy.sh/my-alerts \
   dnswatcher
 ```