feat(admin): multi-select episode file deletion — checkboxes on downloaded rows

The plan's per-EPISODE bulk select for the admin view (§0 item 8
follow-up — trimmed in the first admin-view commit; user called it
back). Admins now tick checkboxes on DOWNLOADED episode rows and delete
their files in one request:

- Checkbox per row, rendered ONLY for admins + downloaded episodes
  (nothing to delete otherwise), via the existing podcastDetail Alpine
  component: selection ids/count survive the list's htmx swaps
  (reapplied on afterSwap — same pattern as the text filter) and clear
  after a successful bulk delete.
- Toolbar segment outside #episode-list (survives swaps): 'N selected'
  + 'Delete selected files' (hx-include gathers the CHECKED boxes);
  disabled-looking at zero selection; hx-confirm states the
  re-materialization contract. CSP-safe method bindings only
  (bulkSelectionLabel/Class) — no ternaries in Alpine expressions.
- POST /feeds/:id/episodes/delete-files (admin): repeated
  episode-file-select keys parsed via ParseForm directly (never bound —
  repeated-key wire is another echo-bind gotcha); each id validated to
  belong to THIS feed, so a forged foreign id is refused while the real
  selections apply; rows reset to pending, files removed.
- Tests: bulk of two real ids + one forged id → deleted:2, files gone,
  rows pending; non-admin 403; all prior single/feed-level pins hold.

internal/router/podcast.go

@@ -78,5 +78,6 @@ func registerPodcastRoutes(cfg *Config) {
 	// Admin maintenance (§0 item 8): file deletion frees the disk and
 	// resets rows to pending. Handler checks admin role.
 	feeds.POST("/delete-files", handlers.AdminMiddleware(cfg.PodcastHandler.AdminDeleteFeedFiles))
+	feeds.POST("/episodes/delete-files", handlers.AdminMiddleware(cfg.PodcastHandler.AdminDeleteEpisodeFilesBulk))
 	feeds.POST("/episodes/:eid/delete-file", handlers.AdminMiddleware(cfg.PodcastHandler.AdminDeleteEpisodeFile))
 }