config: stop startup on an invalid DNS or TLS interval (closes #177)

DNSWATCHER_DNS_INTERVAL and DNSWATCHER_TLS_INTERVAL were parsed with
time.ParseDuration and silently replaced by the default when that failed,
so a value like 5 or 1d gave hourly checks with no hint why, and zero or
negative values were accepted. Both now go through parseInterval, which
returns an error naming the variable and the value, and startup stops the
same way it does for invalid targets. An unset or empty variable still gets
its default from setupViper. The three tests that pinned the old fallback
are replaced. The README says what a valid value looks like.

Model: opus-5-5

internal/config/config.go

@@ -28,6 +28,13 @@ var ErrNoTargets = errors.New(
 	"no monitoring targets configured: set DNSWATCHER_TARGETS environment variable",
 )
 
+// ErrInvalidInterval is returned when DNSWATCHER_DNS_INTERVAL or
+// DNSWATCHER_TLS_INTERVAL is set but is not a positive duration. An empty
+// value counts as unset and means the default.
+var ErrInvalidInterval = errors.New(
+	"interval must be a positive duration such as 30m or 1h",
+)
+
 // Params contains dependencies for Config.
 type Params struct {
 	fx.In
@@ -125,18 +132,14 @@ func buildConfig(
 		}
 	}
 
-	dnsInterval, err := time.ParseDuration(
-		viper.GetString("DNS_INTERVAL"),
-	)
+	dnsInterval, err := parseInterval("DNS_INTERVAL")
 	if err != nil {
-		dnsInterval = defaultDNSInterval
+		return nil, err
 	}
 
-	tlsInterval, err := time.ParseDuration(
-		viper.GetString("TLS_INTERVAL"),
-	)
+	tlsInterval, err := parseInterval("TLS_INTERVAL")
 	if err != nil {
-		tlsInterval = defaultTLSInterval
+		return nil, err
 	}
 
 	domains, hostnames, err := parseAndValidateTargets()
@@ -168,6 +171,22 @@ func buildConfig(
 	return cfg, nil
 }
 
+// parseInterval reads the DNSWATCHER_-prefixed setting key as a duration. A
+// value that does not parse, or is zero or negative, is an error naming the
+// variable and the value; an unset variable has its default from setupViper.
+func parseInterval(key string) (time.Duration, error) {
+	value := viper.GetString(key)
+
+	interval, err := time.ParseDuration(value)
+	if err != nil || interval <= 0 {
+		return 0, fmt.Errorf(
+			"invalid DNSWATCHER_%s %q: %w", key, value, ErrInvalidInterval,
+		)
+	}
+
+	return interval, nil
+}
+
 func parseAndValidateTargets() ([]string, []string, error) {
 	domains, hostnames, err := ClassifyTargets(
 		parseCSV(viper.GetString("TARGETS")),