## Summary
Migrated the dataset document upload API (`POST
/api/v1/datasets/:dataset_id/documents`) from Python to the Go backend.
It supports local file uploads (`type=local`), web page ingestion
(`type=web`), and empty document creation (`type=empty`).
## Changes
- **Router**: Registered `POST /api/v1/datasets/:dataset_id/documents`
route.
- **Handler**: Implemented `UploadDocuments` handler and its routing
functions (`uploadLocalDocuments`, `uploadWebDocument`,
`uploadEmptyDocument`).
- **Service**: Implemented `UploadLocalDocuments`, `UploadWebDocument`,
and `UploadEmptyDocument` in `DocumentService`.
- **Refactoring**: Moved permission checking logic to a shared helper
for reuse in file and document services.
- **Tests**: Added comprehensive unit tests for the new handler and
service upload paths.
## Verification
Ran and passed the test suite for service and handler packages:
- `go test ./internal/service`
- `go test ./internal/handler`
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