Config changes

- a source's interval must be greater than 0: a source is turned off with
  enabled, not with a zero interval, so JSCSourcesSource rejects an interval of
  0 or less (its validate is promoted to each source's section);
- max_message_size rejects -1: JSCServerWebSocket rejects a max_message_size
  of -1 (util.ByteSizeNoLimit), as a client could then exhaust the server's
  memory;
- every setting documents what 0 or empty means (zero/none, no limit or a
  default).

internal/config/conf_sources.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"errors"
 	"reflect"
 	"time"
 )
@@ -39,7 +40,9 @@ func (c *JSCSources) SetValue(key string, value any) error {
 // ========== Generic source ==========
 
 // JSCSourcesSource holds the settings every source has, and is embedded
-// in each source's section. Interval is how often an enabled source fetches.
+// in each source's section. Interval is how often an enabled source fetches;
+// it must be greater than 0 (validate rejects 0 or less: a source is turned
+// off with Enabled, not with its interval).
 type JSCSourcesSource struct {
 	Enabled  bool          `cfg:"enabled"`
 	Interval time.Duration `cfg:"interval" default:"10m"`
@@ -67,3 +70,14 @@ func (c *JSCSourcesSource) SetValue(key string, value any) error {
 
 // Keys returns the keys directly in the section.
 func (c *JSCSourcesSource) Keys() []string { return keysOf(c) }
+
+// validate checks values before they are stored (see validator); it is
+// promoted to each source's section. interval must be greater than 0.
+func (c *JSCSourcesSource) validate(key string, value any) error {
+	if key == "interval" && value.(time.Duration) <= 0 {
+		return errors.New(
+			"interval must be greater than 0; set enabled to false to turn the source off",
+		)
+	}
+	return nil
+}