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_delete_media_items.md

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+# Bulk Delete Media Items
+
+Delete multiple media items at once (supports ebooks, comics, manga).
+
+**Endpoint**: `POST /api/media-items/bulk-delete`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| media_item_ids | array of UUID | Yes | Array of media item UUIDs to delete |
+
+### Example Request
+
+```json
+{
+  "media_item_ids": [
+    "550e8400-e29b-41d4-a716-446655440001",
+    "660e8400-e29b-41d4-a716-446655440002",
+    "770e8400-e29b-41d4-a716-446655440003"
+  ]
+}
+```
+
+## 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"
+    },
+    {
+      "media_item_id": "770e8400-e29b-41d4-a716-446655440003",
+      "status": "success"
+    }
+  ],
+  "total": 3,
+  "deleted": 2,
+  "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 |
+| deleted | number | Number of media items successfully deleted |
+| failed | number | Number of media items that failed to delete |
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request data or empty media_item_ids array |
+| 401 | Invalid or expired token |
+| 403 | User does not have permission |
+| 500 | Server error during deletion |
+
+## Notes
+
+- **File deletion**: Successfully deleted media items will also have their associated files removed from disk
+- **Invalid UUIDs**: Invalid UUID formats are counted as failures in the results array
+- **Non-existent items**: Media items that don't exist are counted as failures (not errors)
+- **Partial success**: The operation continues even if some deletions fail