docs(media-items): Work Order 3 batch 3b — management endpoints (8)

New frontmatter'd docs:
- download (ServeFile — raw bytes, family-lens enforced; corrects the old
  download doc's claim that no download route is registered: ServeFile IS
  routed, DownloadBook is the dead code)
- set_audio_speed (speed range 0.25-4.0, NaN/Inf rejected)
- rescan (abridged example over the full 70+-column row; write-after-success
  guarantee noted)
- purge-archived (destructive, admin), unarchive (204), merge (SHA-256
  precondition, per-user collisions keep active copy)
- facets (two modes: facet+values, field autocomplete counts)
- filter_count (family lens)

Registry: RawMessage pins for map/abridged responses; Schema{} for non-JSON
body routes (download = raw bytes; unarchive = 204). Audit now:
missing docs down to ~55 (from 112), 0 orphans.

docs/developer/api/media-items/purge_archived_media_items.md

@@ -0,0 +1,29 @@
+---
+method: POST
+path: /api/media-items/purge-archived
+auth: jwt+admin
+area: media-items
+---
+<!-- source: internal/handlers/media.go:1747 MediaHandler.PurgeArchivedMediaItems -->
+
+# Purge Archived Media Items
+
+**Admin only.** Permanently deletes ALL archived media items and their
+cover files. This is destructive — archived rows hold no reading data merge
+path (use [Merge Archived Media Item](merge_archived_media_item.md) per item
+first if reading history should survive).
+
+## Example response
+
+```json example-response
+{
+  "purged": 3
+}
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 403 | `{"error": "admin access required"}` | not an admin |
+| 500 | `{"error": "..."}` | unexpected server error |