Batch upload endpoint — accept multiple files

[krapfj23]

Summary

Add an endpoint that accepts multiple files in a single request.

Blocked by

• Research: batch ingestion error handling strategy #80 (batch ingestion research)

Blocks

• Batch processing — concurrent cognify + per-file error reporting #82 (batch concurrent processing)

Steps

1. Add POST /documents/upload-batch to the documents router
2. Accept multiple files via List[UploadFile] and a dataset_name query param
3. Validate each file (type, size, filename) — return 400 with per-file errors if validation fails
4. Save all valid files to temp directory
5. Return 202 Accepted with a list of files queued for processing (or process synchronously if research recommends it)

Acceptance Criteria

• POST /documents/upload-batch accepts multiple files
• Per-file validation with clear error messages
• Tests: upload 3 valid files → accepted, 1 bad file → partial error
• PR body includes Closes #81