config: fail fast when DNSWATCHER_TARGETS is empty

When DNSWATCHER_TARGETS is empty or unset (the default), dnswatcher now
exits with a clear error message instead of silently starting with
nothing to monitor.

Added ErrNoTargets sentinel error returned from config.New when both
domains and hostnames lists are empty after target classification. This
causes the fx application to fail to start, preventing silent
misconfiguration.

Also extracted classifyAndValidateTargets and parseDurationOrDefault
helper functions to keep buildConfig within the funlen limit.

Closes #69

internal/config/config.go

@@ -15,6 +15,12 @@ import (
 	"sneak.berlin/go/dnswatcher/internal/logger"
 )
 
+// ErrNoTargets is returned when DNSWATCHER_TARGETS is empty or unset.
+var ErrNoTargets = errors.New(
+	"no targets configured: set DNSWATCHER_TARGETS to a comma-separated " +
+		"list of DNS names to monitor",
+)
+
 // Default configuration values.
 const (
 	defaultPort             = 8080
@@ -118,25 +124,9 @@ func buildConfig(
 		}
 	}
 
-	dnsInterval, err := time.ParseDuration(
-		viper.GetString("DNS_INTERVAL"),
-	)
+	domains, hostnames, err := classifyAndValidateTargets()
 	if err != nil {
-		dnsInterval = defaultDNSInterval
-	}
-
-	tlsInterval, err := time.ParseDuration(
-		viper.GetString("TLS_INTERVAL"),
-	)
-	if err != nil {
-		tlsInterval = defaultTLSInterval
-	}
-
-	domains, hostnames, err := ClassifyTargets(
-		parseCSV(viper.GetString("TARGETS")),
-	)
-	if err != nil {
-		return nil, fmt.Errorf("invalid targets configuration: %w", err)
+		return nil, err
 	}
 
 	cfg := &Config{
@@ -148,8 +138,8 @@ func buildConfig(
 		SlackWebhook:      viper.GetString("SLACK_WEBHOOK"),
 		MattermostWebhook: viper.GetString("MATTERMOST_WEBHOOK"),
 		NtfyTopic:         viper.GetString("NTFY_TOPIC"),
-		DNSInterval:       dnsInterval,
-		TLSInterval:       tlsInterval,
+		DNSInterval:       parseDurationOrDefault("DNS_INTERVAL", defaultDNSInterval),
+		TLSInterval:       parseDurationOrDefault("TLS_INTERVAL", defaultTLSInterval),
 		TLSExpiryWarning:  viper.GetInt("TLS_EXPIRY_WARNING"),
 		SentryDSN:         viper.GetString("SENTRY_DSN"),
 		MaintenanceMode:   viper.GetBool("MAINTENANCE_MODE"),
@@ -162,6 +152,32 @@ func buildConfig(
 	return cfg, nil
 }
 
+func classifyAndValidateTargets() ([]string, []string, error) {
+	domains, hostnames, err := ClassifyTargets(
+		parseCSV(viper.GetString("TARGETS")),
+	)
+	if err != nil {
+		return nil, nil, fmt.Errorf(
+			"invalid targets configuration: %w", err,
+		)
+	}
+
+	if len(domains) == 0 && len(hostnames) == 0 {
+		return nil, nil, ErrNoTargets
+	}
+
+	return domains, hostnames, nil
+}
+
+func parseDurationOrDefault(key string, fallback time.Duration) time.Duration {
+	d, err := time.ParseDuration(viper.GetString(key))
+	if err != nil {
+		return fallback
+	}
+
+	return d
+}
+
 func parseCSV(input string) []string {
 	if input == "" {
 		return nil