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/set_audio_speed.md

@@ -0,0 +1,37 @@
+---
+method: PUT
+path: /api/media-items/:id/audio-speed
+auth: jwt
+area: media-items
+---
+<!-- source: internal/handlers/media.go:3033 MediaHandler.SetAudioSpeed -->
+
+# Set Audio Speed
+
+Persists the audiobook playback speed for the signed-in user on one media
+item. Speed must be within **0.25–4.0** (NaN/Inf rejected).
+
+## Example request
+
+```json example-request
+{
+  "speed": 1.5
+}
+```
+
+## Example response
+
+```json example-response
+{
+  "playback_speed": 1.5
+}
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "invalid media item id"}` / `{"error": "invalid request"}` / `{"error": "speed out of range"}` | malformed or out-of-range |
+| 401 | `{"error": "authentication required"}` / `{"error": "invalid user"}` | no/invalid auth |
+| 404 | `{"error": "book not found"}` | no such media item |
+| 500 | `{"error": "failed to save playback speed"}` | unexpected server error |