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/handlers/podcast.go

@@ -9,7 +9,6 @@ package handlers
 import (
 	"context"
 	"encoding/json"
-	"fmt"
 	"log"
 	"sort"
 
@@ -39,8 +38,11 @@ type PodcastHandler struct {
 // EpisodeListRenderer renders the post-action episode list for an htmx
 // request. includePolicySelect additionally out-of-band-swaps the
 // auto-download select (Download All flips the caller's policy — the
-// dropdown must reflect it immediately, issue #3).
-type EpisodeListRenderer func(c *echo.Context, feedID string, includePolicySelect bool) error
+// dropdown must reflect it immediately, issue #3);
+// includeOrderSelect likewise rides along for the order endpoint (the
+// "Reverse feed order" button flips to reverse — the dropdown must
+// mirror the server state, §0 item 3).
+type EpisodeListRenderer func(c *echo.Context, feedID string, includePolicySelect, includeOrderSelect bool) error
 
 // SetEpisodeListRenderer wires the renderer (router-only; product wiring).
 func (h *PodcastHandler) SetEpisodeListRenderer(r EpisodeListRenderer) {
@@ -49,9 +51,9 @@ func (h *PodcastHandler) SetEpisodeListRenderer(r EpisodeListRenderer) {
 
 // hxEpisodeList answers an htmx action with the refreshed partial when
 // a renderer is wired; otherwise falls back to 204 + HX-Trigger.
-func (h *PodcastHandler) hxEpisodeList(c *echo.Context, feedID string, includePolicySelect bool) error {
+func (h *PodcastHandler) hxEpisodeList(c *echo.Context, feedID string, includePolicySelect, includeOrderSelect bool) error {
 	if h.episodeListRenderer != nil {
-		return h.episodeListRenderer(c, feedID, includePolicySelect)
+		return h.episodeListRenderer(c, feedID, includePolicySelect, includeOrderSelect)
 	}
 	return hxAccepted(c, "")
 }
@@ -328,7 +330,7 @@ func (h *PodcastHandler) UpdateSubscription(c *echo.Context) error {
 	if isHXRequest(c) {
 		// Policy changes feed the auto-download union on the next poll.
 		_, _ = h.worker.ApplyAutoPolicies(ctx, feedID)
-		return h.hxEpisodeList(c, feedID.String(), true)
+		return h.hxEpisodeList(c, feedID.String(), true, false)
 	}
 	return c.JSON(http.StatusOK, updated)
 }
@@ -367,7 +369,7 @@ func (h *PodcastHandler) RefreshFeed(c *echo.Context) error {
 		return podcastErrorResponse(c, err)
 	}
 	if isHXRequest(c) {
-		return h.hxEpisodeList(c, feedID.String(), false)
+		return h.hxEpisodeList(c, feedID.String(), false, false)
 	}
 	return c.JSON(http.StatusOK, map[string]string{"message": "refreshed"})
 }
@@ -418,7 +420,7 @@ func (h *PodcastHandler) DownloadEpisodes(c *echo.Context) error {
 			// Download All flips the caller's policy to all (issue #3)
 			// — the oob select keeps the dropdown honest.
 			h.flipSubscriptionToAll(ctx, user.ID, feedID)
-			return h.hxEpisodeList(c, feedID.String(), true)
+			return h.hxEpisodeList(c, feedID.String(), true, false)
 		}
 		return c.JSON(http.StatusAccepted, map[string]interface{}{"queued": queued})
 	}
@@ -430,7 +432,7 @@ func (h *PodcastHandler) DownloadEpisodes(c *echo.Context) error {
 		return podcastErrorResponse(c, err)
 	}
 	if isHXRequest(c) {
-		return h.hxEpisodeList(c, feedID.String(), false)
+		return h.hxEpisodeList(c, feedID.String(), false, false)
 	}
 	return c.JSON(http.StatusAccepted, map[string]string{"message": "queued"})
 }
@@ -503,9 +505,10 @@ func podcastOrderSettingKey(feedID string) string {
 	return "podcast_order:" + feedID
 }
 
-// podcastOrderValid: native (feed position) | oldest | newest.
+// podcastOrderValid native (feed position) | reverse (position DESC,
+// no dates) | oldest | newest.
 func podcastOrderValid(o string) bool {
-	return o == "native" || o == "oldest" || o == "newest"
+	return services.ValidPodcastEpisodeOrder(o)
 }
 
 // GetFeedOrder reads the caller's effective order for a feed
@@ -538,7 +541,7 @@ func (h *PodcastHandler) SetFeedOrder(c *echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid feed id"})
 	}
 	var req struct {
-		Order string `json:"order" form:"order" validate:"required,oneof=native oldest newest"`
+		Order string `json:"order" form:"order" validate:"required,oneof=native reverse oldest newest"`
 	}
 	if err := c.Bind(&req); err != nil {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid request"})
@@ -564,7 +567,7 @@ func (h *PodcastHandler) SetFeedOrder(c *echo.Context) error {
 		}
 	}
 	if isHXRequest(c) {
-		return h.hxEpisodeList(c, feedID.String(), false)
+		return h.hxEpisodeList(c, feedID.String(), false, true)
 	}
 	return c.JSON(http.StatusOK, map[string]string{"order": req.Order})
 }
@@ -590,19 +593,11 @@ func (h *PodcastHandler) GetFeedSequence(c *echo.Context) error {
 	for i := range eps {
 		views = append(views, episodeView(&eps[i]))
 	}
-	sortSlice(views, func(a, b PodcastEpisodeView) bool {
-		switch order {
-		case "oldest":
-			return episodeSortKey(a) < episodeSortKey(b)
-		case "newest":
-			return episodeSortKey(a) > episodeSortKey(b)
-		default: // native: feed position (fetch-maintained)
-			if a.FeedPosition != b.FeedPosition {
-				return a.FeedPosition < b.FeedPosition
-			}
-			return a.Date < b.Date
-		}
-	})
+	// Effective listening order (order consolidation §0 item 5a).
+	services.OrderEpisodes(views, order,
+		func(v PodcastEpisodeView) int32 { return int32(v.FeedPosition) },
+		func(v PodcastEpisodeView) string { return v.Date },
+	)
 
 	sequence := make([]PodcastEpisodeView, 0, len(views))
 	gaps := make([]PodcastEpisodeView, 0)
@@ -621,12 +616,6 @@ func (h *PodcastHandler) GetFeedSequence(c *echo.Context) error {
 	})
 }
 
-// episodeSortKey: chronological key for oldest/newest (date, then
-// native position as the same-day tiebreaker).
-func episodeSortKey(v PodcastEpisodeView) string {
-	return v.Date + fmt.Sprintf("%06d", v.FeedPosition)
-}
-
 // sortSlice: sort.Slice with a controller-friendly wrapper.
 func sortSlice[T any](s []T, less func(a, b T) bool) {
 	sort.Slice(s, func(i, j int) bool { return less(s[i], s[j]) })