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/get_unlinked_book_suggestions.md

@@ -0,0 +1,71 @@
+# Get Unlinked Book Suggestions
+
+Get suggested matches for unlinked books on a device.
+
+**Endpoint**: `GET /api/sync/unlinked-books/:id/suggestions`
+**Auth**: Required
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| id | string (UUID) | Yes | Device UUID |
+
+## Query Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| limit | integer | No | Maximum number of suggestions per book (default: 5) |
+| threshold | float | No | Minimum confidence threshold (default: 0.5) |
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token |
+
+### Example Request
+
+```http
+GET /api/sync/unlinked-books/550e8400-e29b-41d4-a716-446655440000/suggestions?limit=3&threshold=0.6
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (200 OK)
+
+```json
+{
+  "suggestions": [
+    {
+      "book_id": "book-123",
+      "book_title": "Foundation",
+      "book_author": "Isaac Asimov",
+      "matches": [
+        {
+          "media_item_id": "uuid",
+          "title": "Foundation",
+          "authors": ["Isaac Asimov"],
+          "confidence": 0.95
+        },
+        {
+          "media_item_id": "uuid-2",
+          "title": "Foundation and Empire",
+          "authors": ["Isaac Asimov"],
+          "confidence": 0.75
+        }
+      ]
+    }
+  ]
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token |
+| 404 | Device not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->