docs(api): add new endpoint documentation

- Add create_media_item.md for media item creation API

- Add delete_rating.md for rating deletion endpoint

- Add update_rating.md for rating update endpoint

docs/developer/api/ratings/delete_rating.md

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+# Delete Rating
+
+Delete the current user's rating for a media item.
+
+**Endpoint**: `DELETE /api/media-items/{media_id}/rating`
+**Auth**: Required
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| media_id | string | Yes | Media item UUID |
+
+### Example Request
+
+```http
+DELETE /api/media-items/550e8400-e29b-41d4-a716-446655440000/rating
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (200 OK)
+
+```json
+{
+  "message": "Rating deleted successfully"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token |
+| 404 | Media item or rating not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/developer/api/ratings/update_rating.md

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+# Update Rating
+
+Update an existing rating for a media item.
+
+**Endpoint**: `PUT /api/media-items/{media_id}/rating`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| media_id | string | Yes | Media item UUID |
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| rating | integer | Yes | Rating value (1-10 scale) |
+| review | string | No | Optional review text |
+
+**Rating Scale**: 1-10 (odd numbers = half-stars: 1=0.5★, 2=1★, 3=1.5★, ..., 9=4.5★, 10=5★)
+
+### Example Request
+
+```json
+{
+  "rating": 8,
+  "review": "Great book! Very enjoyable read."
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "id": "uuid",
+  "media_item_id": "uuid",
+  "user_id": "uuid",
+  "rating": 8,
+  "review": "Great book! Very enjoyable read.",
+  "created_at": "2026-01-31T10:00:00Z",
+  "updated_at": "2026-01-31T11:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request data |
+| 401 | Invalid or expired token |
+| 404 | Media item or rating not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->