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/reader.go

@@ -9,9 +9,7 @@ import (
 	"bookhoard/templates"
 	"bytes"
 	"encoding/json"
-	"fmt"
 	"net/http"
-	"sort"
 
 	"github.com/google/uuid"
 	"github.com/jackc/pgx/v5/pgtype"
@@ -186,7 +184,7 @@ func buildPodcastSequenceContext(c *echo.Context, cfg *Config, item *database.Me
 		var v struct {
 			Order string `json:"order"`
 		}
-		if json.Unmarshal(row, &v) == nil && (v.Order == "oldest" || v.Order == "newest") {
+		if json.Unmarshal(row, &v) == nil && services.ValidPodcastEpisodeOrder(v.Order) {
 			order = v.Order
 		}
 	}
@@ -215,17 +213,11 @@ func buildPodcastSequenceContext(c *echo.Context, cfg *Config, item *database.Me
 			pos:   ep.FeedPosition.Int32,
 		})
 	}
-	sort.SliceStable(ordered, func(a, b int) bool {
-		ka := ordered[a].date + fmt.Sprintf("%06d", ordered[a].pos)
-		kb := ordered[b].date + fmt.Sprintf("%06d", ordered[b].pos)
-		if order == "newest" {
-			return ka > kb
-		}
-		if order == "oldest" {
-			return ka < kb
-		}
-		return ordered[a].pos < ordered[b].pos
-	})
+	// Effective listening order (order consolidation §0 item 5a).
+	services.OrderEpisodes(ordered, order,
+		func(e seqEp) int32 { return e.pos },
+		func(e seqEp) string { return e.date },
+	)
 
 	idx := -1
 	for i, ep := range ordered {