Add persistent CSV/JSON uploads and improve upload workflow

[saranshkr]

Summary

This PR adds CSV and JSON upload support, persists uploaded data per user, and improves the upload workflow across the app.

What changed

• added support for uploading tabular CSV/TSV data and nested JSON data
• normalized uploaded files into a registry-backed structure for planner/executor use
• persisted upload metadata in SQLite and linked each upload to the signed-in user
• stored raw uploaded files on the local filesystem
• kept DuckDB as the normalized query layer for uploaded sources
• added authenticated upload listing so files reappear after refresh and after logging out and back in
• restricted upload visibility and analysis access to the owning user
• made chat use saved uploads even when files were uploaded from the Uploads page
• added delete actions for uploaded files
• removed the Recent uploads sidebar block to avoid layout issues and keep the sidebar stable

Testing

• ran .venv/bin/pytest tests/test_auth.py tests/test_api.py tests/test_source_registry.py tests/test_planner_schema.py tests/test_tools.py
• verified CSV and JSON uploads are ingested correctly
• verified uploads persist across refresh/login
• verified users only see and analyze their own uploads
• verified uploaded files can be deleted cleanly

[abhinavsingh9714]

LGTM