docs: add ratings, devices, and analytics API endpoints

Phase 2 part 5: Split device management and analytics endpoints
- Ratings: get_ratings, create_rating (1-10 scale with half-stars)
- Devices: register_device, list_devices, get_devices, revoke_device
- Analytics: get_analytics (reading statistics)
- Include device registration flow details

docs/api/devices/register_device.md

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+# Register Device
+
+Register a new device for sync.
+
+**Endpoint**: `POST /api/devices/register`
+**Auth**: Not required (device registration flow)
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| device_name | string | Yes | Device name |
+| device_type | string | Yes | Device type: kobo, koreader, web, mobile |
+| device_identifier | string | Yes | Hardware-specific ID |
+
+### Example Request
+
+```json
+{
+  "device_name": "My Kobo Clara",
+  "device_type": "kobo",
+  "device_identifier": "hardware-specific-id"
+}
+```
+
+## Response (201 Created)
+
+```json
+{
+  "device_id": "uuid",
+  "registration_id": "registration-uuid",
+  "auth_url": "https://bookhoard.com/devices/auth/confirm/abc123",
+  "qr_code": "data:image/png;base64,iVBORw0KG...",
+  "expires_in": 300
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid device data |
+| 409 | Device already registered |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->