docs(api): add books bulk operations and download documentation

- Add bulk_delete_books.md for POST /api/books/bulk-delete
- Add bulk_update_books.md for POST /api/books/bulk-update
  - Includes tag/contributor normalization details
  - Documents dual-field normalization behavior
- Add download_book.md for GET /api/books/:uuid/download
  - Public endpoint with auth for non-public libraries
  - Documents Content-Type headers for different formats

Completes books operations API section

docs/developer/api/admin/update_user_max_devices.md

@@ -0,0 +1,53 @@
+# Update User Max Devices
+
+Update the maximum number of devices a user can register (admin only).
+
+**Endpoint**: `PUT /api/auth/users/:id/max-devices`
+**Auth**: Required (Admin only)
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| id | string (UUID) | Yes | User UUID |
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| max_devices | integer | Yes | Maximum number of devices (1-100) |
+
+### Example Request
+
+```json
+{
+  "max_devices": 15
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "id": "uuid",
+  "email": "user@example.com",
+  "username": "johndoe",
+  "max_devices": 15,
+  "active_devices": 3,
+  "updated_at": "2026-02-08T11:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid max_devices value (must be 1-100) |
+| 401 | Invalid or expired token |
+| 403 | User does not have admin privileges |
+| 404 | User not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->