fix(docs): Work Order 2 — correct stale paths (11 docs + 3 Bruno)

Fix all wrong-path artifacts identified by the audit:
- Kobo sync endpoints (init/bookmark/markup/sync-from-server/analytics): add
  missing :token device segment (6 doc files incl. kobo-protocol.md)
- Highlights/notes update+delete: add missing /:id media-item prefix
- media-items/download_media_item.md: removed (static /uploads route, not an
  API endpoint; info about the file route is recoverable from OPDS docs)
- Bruno: /libraries/visible -> /visibility, /collections/books missing id,
  /audio/0 -> track_index
- operations/troubleshooting.md: same /libraries/visible -> /visibility fix
- media-items/get_audio_track.md + get_audio_chapters.md: /audio/0 examples fixed
- Deleted monolithic docs/developer/api-reference.md (duplicate of split docs)
- Fixed 4 stale entries in docs/developer/api/api-reference.md (split index)

Audit re-run: orphan docs 0, orphan Bruno 6 (all scenario files with literal IDs).

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

@@ -1,48 +0,0 @@
-# Download Media Item
-
-Download a media item file (EPUB, PDF, etc.) from the Bookhoard server.
-
-Book files are served by the authenticated file route, the same one the web reader uses. Build the URL from the media item's `library_id` and relative `file_path` (both returned by the media item list/get endpoints):
-
-**Endpoint**: `GET /uploads/library-{library_id}/{file_path}`
-**Auth**: Required (JWT - Bearer header or session cookie)
-
-The `file_path` segments are URL-escaped individually; slashes are preserved. `cover_image_path` uses the same route.
-
-## Path Parameters
-
-| Parameter   | Type   | Required | Description                          |
-| ----------- | ------ | -------- | ------------------------------------ |
-| library_id  | string | Yes      | Library UUID (the item's library)    |
-| file_path   | string | Yes      | The item's relative `file_path`      |
-
-## Response
-
-**Success (200 OK)**: Binary file data
-
-**Response Headers**:
-
-- `Content-Type`: MIME type by file extension (`application/epub+zip`, `application/pdf`, …; `application/octet-stream` fallback)
-- `Cache-Control`: `public, max-age=86400`
-
-## Error Responses
-
-| Code | Description                 |
-| ---- | --------------------------- |
-| 400  | Invalid library ID or path  |
-| 401  | Missing/invalid token       |
-| 404  | File not found on disk      |
-
-## Example
-
-```bash
-curl -O -H "Authorization: Bearer $TOKEN" \
-  "http://localhost:8765/uploads/library/550e8400-.../books/1984.epub"
-```
-
-(URL shape: `/uploads/library-{uuid}/{escaped-relative-path}`.)
-
-## Notes
-
-- **Do not rely on `GET /api/media-items/:id/download`** — it appears in older docs but is **not registered**; `MediaHandler.DownloadBook` exists as dead code. Use the file route above.
-- OPDS-capable devices may alternatively use the device-authenticated `GET /opds/devices/{deviceId}/download/{bookId}`, which supports on-the-fly format conversion (epub, kepub, pdf, cbz).

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

@@ -23,7 +23,7 @@ which file model produced them.
   "tracks": [
     {
       "index": 0,
-      "url": "/api/media-items/{id}/audio/0",
+      "url": "/api/media-items/{id}/audio/{trackIndex}",
       "title": "01 First",
       "path": "Author/Title/01 First.mp3",
       "start": 0.0,

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

@@ -40,5 +40,5 @@ answers **403** on every track (same gate as the download endpoint).
 ```bash
 curl -H "Authorization: Bearer $TOKEN" \
   -H "Range: bytes=0-1023" \
-  "http://localhost:8765/api/media-items/550e8400-.../audio/0"
+  "http://localhost:8765/api/media-items/550e8400-.../audio/{track_index}"
 ```