Add batch delete documents endpoint (closes #78)

[chadvik88]

• New DELETE/documents/batch endpoint accepting a list of document_ids
• Reuses the existing document_service.delete_document logic per document (consistent with how delete_document already works)
• Returns a BatchDeleteResponse with deleted and errors lists, so partial failures are reported per-document rather than failing the whole batch
• Note: SDK, shell, and UI support for this endpoint aren't included yet — happy to follow up with those if this approach looks good

[CLAassistant]

All committers have signed the CLA.

[chadvik88]

Hi again,

The gitleaks check is failing here (and most likely on all PRs) because it's missing a GITLEAKS_LICENSE org secret, not because of anything in this diff. The error is: missing gitleaks license... store it as a GitHub Secret named GITLEAKS_LICENSE. I think it might be worth adding that secret or disabling the check for now.