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/rescan_media_item.md

@@ -0,0 +1,43 @@
+---
+method: POST
+path: /api/media-items/:id/rescan
+auth: jwt+admin
+area: media-items
+---
+<!-- source: internal/handlers/media.go:1765 MediaHandler.RescanMediaItem -->
+
+# Rescan Media Item
+
+Re-extracts metadata for one media item from its file and returns the updated
+row (200, full `media_items` record — 70+ columns; the most relevant are shown
+below).
+
+Write-after-success guarantee: a failed/empty extraction never clears
+scan-derived columns.
+
+## Example response (abridged)
+
+```json example-response
+{
+  "id": "550e8400-e29b-41d4-a716-446655440000",
+  "library_id": "990e8400-e29b-41d4-a716-446655440000",
+  "title": "1984",
+  "author": "George Orwell",
+  "file_path": "/books/1984.epub",
+  "mime_type": "application/epub+zip",
+  "scan_status": "completed",
+  "updated_at": "2026-10-24T14:57:01Z"
+}
+```
+
+The example is abridged for readability — the response carries the full
+`media_items` row.
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "invalid media item id"}` | malformed id |
+| 403 | `{"error": "admin access required"}` | not an admin |
+| 404 | `{"error": "media item not found"}` | no such item |
+| 500 | `{"error": "..."}` | extraction/DB failure |