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

internal/config/config.go

@@ -89,8 +89,7 @@ func setupViper(name string) {
 	viper.SetDefault("PORT", defaultPort)
 	viper.SetDefault("DEBUG", false)
 	viper.SetDefault("DATA_DIR", "./data")
-	viper.SetDefault("DOMAINS", "")
-	viper.SetDefault("HOSTNAMES", "")
+	viper.SetDefault("TARGETS", "")
 	viper.SetDefault("SLACK_WEBHOOK", "")
 	viper.SetDefault("MATTERMOST_WEBHOOK", "")
 	viper.SetDefault("NTFY_TOPIC", "")
@@ -133,12 +132,19 @@ func buildConfig(
 		tlsInterval = defaultTLSInterval
 	}
 
+	domains, hostnames, err := ClassifyTargets(
+		parseCSV(viper.GetString("TARGETS")),
+	)
+	if err != nil {
+		return nil, fmt.Errorf("invalid targets configuration: %w", err)
+	}
+
 	cfg := &Config{
 		Port:              viper.GetInt("PORT"),
 		Debug:             viper.GetBool("DEBUG"),
 		DataDir:           viper.GetString("DATA_DIR"),
-		Domains:           parseCSV(viper.GetString("DOMAINS")),
-		Hostnames:         parseCSV(viper.GetString("HOSTNAMES")),
+		Domains:           domains,
+		Hostnames:         hostnames,
 		SlackWebhook:      viper.GetString("SLACK_WEBHOOK"),
 		MattermostWebhook: viper.GetString("MATTERMOST_WEBHOOK"),
 		NtfyTopic:         viper.GetString("NTFY_TOPIC"),