Submit through background jobs

[msaroufim]

Summary

• Submit test, benchmark, and leaderboard runs through KernelBot's background submission endpoint
• Poll the authenticated user submission endpoint until the server-side job finishes
• Avoid tying long QR leaderboard runs to one long-lived SSE HTTP request

Prod row 814445 had a submission row but no runs and no job status. That matches the old synchronous endpoint path being cancelled or disconnected before Modal returned.

Test

• cargo fmt --check
• cargo check
• cargo test