赵月辉 / fastgpt-migrated

* feat: add metadata field to dataset_data for custom metadata support\n\n- Zod schemas: add metadata to DatasetDataSchema, DatasetDataItemSchema, CreateDatasetDataPropsSchema, UpdateDatasetDataPropsSchema, PushDataChunkSchema, GetDataListItemSchema\n- MongoDB schemas: add metadata to dataset_datas and dataMetadata to dataset_trainings\n- CSV import: parse header row to identify non-q/a/indexes columns as metadata keys\n- Training pipeline: pass metadata through training queue (dataMetadata) to dataset_data (metadata)\n- Search output: include metadata in search result items via buildSearchResultItem\n- API: include metadata in v2/list responses\n- Template: update download template header to demo metadata column, relax header validation to check only first two columns are q,a\n\nCo-Authored-By: Claude <noreply@anthropic.com>

* fix: preserve dataset metadata across import and export

* chore: remove metadata design document

* fix: allow a single metadata template column

* fix: reject legacy dataset template headers

* docs: add dataset metadata release note

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Co-authored-by: hexiaoyan30010 <30010@sangfor.com>
Co-authored-by: Archer <545436317@qq.com>