Increase PHP upload limits and add large dataset upload docs

[lucifer4330k]

This PR increases server-side upload limits and adds documentation to support large dataset uploads.

Changes:

• Set upload_max_filesize to 5G in docker/config/php.ini
• Set post_max_size to 5G in docker/config/php.ini
• Increase max_execution_time to 3600 seconds
• Add LARGE_DATASET_UPLOAD_FIX.md with full guidance
• Add QUICK_FIX_OVERFLOW_ERROR.md with immediate steps
• Update docker/README.md with upload limits section

Rationale:

• Users encounter client-side OverflowError: string longer than 2147483647 bytes when uploading very large datasets due to single-buffer SSL writes.
• Server defaults (2M/8M) also blocked uploads.

This PR documents client-side streaming/compression/external-hosting workarounds and raises server limits to accommodate large datasets.

Fixes #1223