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/operations/troubleshooting.md

@@ -168,10 +168,10 @@ podman-compose ps  # or: docker compose ps
 podman-compose exec db psql -U postgres -d bookhoard -c "SELECT 1;"  # or: docker compose exec db ...
 
 # Test API endpoint:
-curl -s http://localhost:8765/api/libraries/visible
+curl -s http://localhost:8765/api/libraries/visibility
 
 # For remote servers, use actual IP:
-curl -s http://SERVER_IP:8765/api/libraries/visible
+curl -s http://SERVER_IP:8765/api/libraries/visibility
 ```
 
 ## 🚀 Quick Verification
@@ -186,7 +186,7 @@ podman-compose ps  # or: docker compose ps
 podman-compose exec db psql -U postgres -d bookhoard -c "SELECT 1;"  # or: docker compose exec db ...
 
 # Test API endpoint
-curl -s http://localhost:8765/api/libraries/visible
+curl -s http://localhost:8765/api/libraries/visibility
 
 # Check application logs
 docker compose logs app
@@ -209,7 +209,7 @@ docker compose up -d
 
 # 4. Verify
 docker compose ps
-curl -s http://localhost:8765/api/libraries/visible
+curl -s http://localhost:8765/api/libraries/visibility
 ```
 
 ## 🌐 Production Deployment
@@ -319,7 +319,7 @@ The current setup follows best practices:
 When properly set up, you should see:
 
 1. **Healthy containers**: `docker compose ps` shows "Up" status
-2. **Working API**: `curl http://localhost:8765/api/libraries/visible` returns JSON
+2. **Working API**: `curl http://localhost:8765/api/libraries/visibility` returns JSON
 3. **New tables**: `media_highlights` and `media_notes` exist with proper schema
 4. **No port conflicts**: Application binds to port 8765 without errors
 5. **Database ready**: PostgreSQL accepts connections and serves requests