docs(kobo): mark native Kobo sync as coming soon

Native Kobo sync is implemented server-side but not yet supported on
real devices, so stop documenting it as a working feature.

- Rewrite kobo-setup.md as a coming-soon stub: point users to KOReader
  (which runs on Kobo hardware) as the supported path today, and list
  what native sync will deliver when released
- Add 'Coming Soon' status banners to the Kobo protocol spec, all five
  Kobo endpoint docs, and both API references, noting the endpoints are
  under active development and may change
- Tag the device shelf endpoints as pending native Kobo support

docs/developer/api/api-reference.md

@@ -23,7 +23,7 @@ Complete reference for Bookhoard REST API endpoints.
 - [Scanner](scanner/) - Library scanning and watch mode (admin)
 - [OPDS](opds/) - Open Publication Distribution
 - [KOReader](koreader/) - KOReader sync protocol
-- [Kobo](kobo/) - Kobo sync protocol
+- [Kobo](kobo/) - Kobo sync protocol (coming soon)
 - [WebSocket](websocket/) - Real-time sync events
 
 ---
@@ -134,7 +134,7 @@ See [Device Registration & Sync](devices/)
 - GET /api/devices/pending - List pending registrations (admin)
 - GET /api/devices/approve/:registration_id - Approve registration (admin)
 - POST /api/devices/reject/:registration_id - Reject registration (admin)
-- POST /api/devices/:id/shelves - Add to shelf (Kobo)
+- POST /api/devices/:id/shelves - Add to shelf (Kobo; used by native Kobo sync, coming soon)
 - GET /api/devices/:id/shelves - Get shelf contents
 - DELETE /api/devices/:id/shelves - Remove from shelf
 - DELETE /api/devices/:id/shelves/clear - Clear shelf
@@ -226,6 +226,8 @@ See [KOReader Sync](koreader/) and [Sync Protocol](sync/koreader-protocol.md)
 
 ## Kobo Sync Protocol
 
+> **Status: Coming Soon** — Native Kobo sync is implemented server-side but not yet supported on real devices. These endpoints are under active development and may change.
+
 See [Kobo Sync](kobo/) and [Sync Protocol](sync/kobo-protocol.md)
 
 - POST /api/sync/kobo/markup - Sync markup highlights