docs(api): add universal progress tracking documentation

- get_universal_progress.md - GET /api/progress/:id
- update_universal_progress.md - POST /api/progress/:id
- get_progress_history.md - GET /api/progress/:id/history

Documents device-agnostic (universal) reading progress tracking
that works across all devices (Kobo, KOReader, etc.)

docs/developer/api/book-matching/auto_link_books.md

@@ -0,0 +1,52 @@
+# Auto Link Books
+
+Automatically link books to media items based on matching metadata.
+
+**Endpoint**: `POST /api/sync/auto-link-books`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| device_id | string (UUID) | Yes | Device UUID |
+| threshold | float | No | Match confidence threshold (0.0-1.0, default: 0.7) |
+
+### Example Request
+
+```json
+{
+  "device_id": "550e8400-e29b-41d4-a716-446655440000",
+  "threshold": 0.8
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "message": "Auto-link completed",
+  "linked_count": 15,
+  "unmatched_count": 5,
+  "links": [
+    {
+      "book_id": "book-123",
+      "media_item_id": "uuid",
+      "confidence": 0.95
+    }
+  ]
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request data |
+| 401 | Invalid or expired token |
+| 404 | Device not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->