feat(podcasts): reverse order — position DESC, date-agnostic; single order helper

The 'Oldest first' order sorts by PUB DATE, which breaks on Scribl's
migrated podiobooks feeds (messy/all-equal dates). New order value
'reverse' = feed position DESC with a date-DESC tiebreak — dates are
not involved, so nothing can break.

- services.OrderEpisodes: THE single order source of truth (§0 items
  3+5a). The four surfaces (sequence JSON, reader chain context, SSR
  page list) previously carried three near-identical handwritten
  comparators — all now route through one generic helper. The SSR page
  list gains a SortDate key (lexical 2006-01-02) so the pub-date
  tiebreak is actually chronological; the old comparator compared
  'Jan 2, 2006' display strings alphabetically (latent drift bug).
- Dropdown becomes four options: Feed order / Feed order (reversed) /
  Oldest first (pub date) / Newest first (pub date).
- Serial suggestion button renamed 'Reverse feed order' (sets reverse,
  not oldest) and its response out-of-band-swaps the order select
  (PodcastOrderSelectOOB — same mechanism as Download All's policy
  select), so the dropdown mirrors the server state immediately.
- Validation: oneof=native reverse oldest newest everywhere.
- Tests: reverse binds via the form wire; messy-dates sequence
  returns 3,2,1; existing reorder-safety stays green.

internal/router/podcast.go

@@ -20,7 +20,7 @@ func registerPodcastRoutes(cfg *Config) {
 	// refreshed partial directly — no event indirection to miss. The
 	// view assembly lives here (templates) while the action logic lives
 	// in the handler; this closure is the sanctioned seam.
-	cfg.PodcastHandler.SetEpisodeListRenderer(func(c *echo.Context, feedID string, includePolicySelect bool) error {
+	cfg.PodcastHandler.SetEpisodeListRenderer(func(c *echo.Context, feedID string, includePolicySelect, includeOrderSelect bool) error {
 		user, err := getTemplateUserWithTheme(c, cfg)
 		if err != nil {
 			return err
@@ -48,6 +48,14 @@ func registerPodcastRoutes(cfg *Config) {
 				return err
 			}
 		}
+		if includeOrderSelect {
+			// §0 item 3: out-of-band swap of the order select — the
+			// "Reverse feed order" button flips to reverse; the
+			// dropdown mirrors the server state immediately.
+			if err := templates.PodcastOrderSelectOOB(data.Feed.ID, data.Order).Render(c.Request().Context(), &buf); err != nil {
+				return err
+			}
+		}
 		return c.HTML(http.StatusOK, buf.String())
 	})