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.

templates/podcast_detail.templ

@@ -116,21 +116,24 @@ templ PodcastDetailPage(user User, data PodcastDetailData) {
 						hx-swap="outerHTML"
 					>
 						<option value="native" selected={ data.Order == "native" }>Feed order</option>
-						<option value="oldest" selected={ data.Order == "oldest" }>Oldest first</option>
-						<option value="newest" selected={ data.Order == "newest" }>Newest first</option>
+						<option value="reverse" selected={ data.Order == "reverse" }>Feed order (reversed)</option>
+						<option value="oldest" selected={ data.Order == "oldest" }>Oldest first (pub date)</option>
+						<option value="newest" selected={ data.Order == "newest" }>Newest first (pub date)</option>
 					</select>
 
-					<!-- Serial suggestion (D5): one-click, never silent -->
+					<!-- Serial suggestion (D5, §0 item 3 rename): one-click,
+					     never silent. "reverse" sorts by FEED POSITION DESC —
+					     no pub dates involved, so messy dates can't break it. -->
 					if data.IsSerial && data.Order == "native" {
 						<button
 							class="btn btn-secondary text-sm"
 							hx-put={ "/api/podcasts/feeds/" + data.Feed.ID + "/order" }
-							hx-vals='{"order": "oldest"}'
+							hx-vals='{"order": "reverse"}'
 							hx-target="#episode-list"
 							hx-swap="outerHTML"
-							title="Serial feed: publishers list newest first — listen oldest-first"
+							title="Serial feed: publishers list newest first — reverse the feed order to start at chapter 1"
 						>
-							<span>Serial? Listen oldest-first</span>
+							<span>Reverse feed order</span>
 						</button>
 					}
 
@@ -382,6 +385,27 @@ templ PodcastAutoDownloadSelect(feed PodcastFeedCard) {
 	</select>
 }
 
+// PodcastOrderSelectOOB is the out-of-band variant of the order
+// dropdown: same element id + hx-swap-oob so htmx replaces the
+// action-row select in place (§0 item 3 — the "Reverse feed order"
+// button flips the order server-side; the dropdown must show it).
+templ PodcastOrderSelectOOB(feedID string, order string) {
+	<select
+		id="podcast-order-select"
+		name="order"
+		hx-swap-oob="true"
+		class="input w-auto py-1.5 cursor-pointer"
+		hx-put={ "/api/podcasts/feeds/" + feedID + "/order" }
+		hx-target="#episode-list"
+		hx-swap="outerHTML"
+	>
+		<option value="native" selected={ order == "native" }>Feed order</option>
+		<option value="reverse" selected={ order == "reverse" }>Feed order (reversed)</option>
+		<option value="oldest" selected={ order == "oldest" }>Oldest first (pub date)</option>
+		<option value="newest" selected={ order == "newest" }>Newest first (pub date)</option>
+	</select>
+}
+
 // PodcastAutoDownloadSelectOOB is the out-of-band variant: same element
 // id + hx-swap-oob so htmx replaces the action-row select in place.
 templ PodcastAutoDownloadSelectOOB(feed PodcastFeedCard) {