docs: add media-items bulk operation and download documentation

- Add comprehensive documentation for bulk delete endpoint
- Add comprehensive documentation for bulk update endpoint
- Add comprehensive documentation for download endpoint
- Document all request/response fields with correct names
- Include examples and error codes
- Add notes on tag normalization and partial success

docs/developer/api/media-items/bulk_update_media_items.md

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+# Bulk Update Media Items
+
+Update multiple media items at once (supports ebooks, comics, manga).
+
+**Endpoint**: `POST /api/media-items/bulk-update`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| media_item_updates | array of objects | Yes | Array of update operations |
+| media_item_updates[].media_item_id | string (UUID) | Yes | Media item UUID to update |
+| media_item_updates[].updates | object | Yes | Fields to update |
+
+### Update Fields
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| title | string | No | Updated title |
+| author | string | No | Updated author |
+| genre | string | No | Updated genre |
+| language | string | No | Updated language (ISO 639-1 code) |
+| tags | array of strings | No | Updated tags (auto-normalized) |
+
+### Example Request
+
+```json
+{
+  "media_item_updates": [
+    {
+      "media_item_id": "550e8400-e29b-41d4-a716-446655440001",
+      "updates": {
+        "title": "Updated Title",
+        "genre": "Science Fiction",
+        "tags": ["science fiction", "space opera", "ACME CORP."]
+      }
+    },
+    {
+      "media_item_id": "660e8400-e29b-41d4-a716-446655440002",
+      "updates": {
+        "author": "Updated Author Name",
+        "language": "en"
+      }
+    }
+  ]
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "results": [
+    {
+      "media_item_id": "550e8400-e29b-41d4-a716-446655440001",
+      "status": "success"
+    },
+    {
+      "media_item_id": "660e8400-e29b-41d4-a716-446655440002",
+      "status": "error",
+      "error": "Media item not found"
+    }
+  ],
+  "total": 2,
+  "updated": 1,
+  "failed": 1
+}
+```
+
+## Response Fields
+
+| Field | Type | Description |
+|-------|------|-------------|
+| results | array | Individual result for each media item |
+| results[].media_item_id | string | UUID of the media item |
+| results[].status | string | "success" or "error" |
+| results[].error | string | Error message (only present if status is "error") |
+| total | number | Total number of media items processed |
+| updated | number | Number of media items successfully updated |
+| failed | number | Number of media items that failed to update |
+
+## Tag and Contributor Normalization
+
+The backend automatically normalizes tags:
+
+- **Tags**: Titlecased, punctuation preserved, deduplicated
+- **Search fields**: Lowercase, no punctuation, stored in `tags_search`
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request data or empty media_item_updates array |
+| 401 | Invalid or expired token |
+| 403 | User does not have permission |
+| 404 | One or more media items not found |
+| 500 | Server error during update |
+
+## Notes
+
+- **Partial success**: The operation continues even if some updates fail
+- **Invalid UUIDs**: Invalid UUID formats are counted as failures
+- **Non-existent items**: Media items that don't exist are counted as failures
+- **No changes**: If updates object is empty, the item still counts as success