fix(team): persist models and queue background messages

[Inference1]

Summary

• persist confirmed Team model changes to the Team roster and conversation runtime seed
• keep an active Team session in sync with the confirmed model
• queue manual and lead-initiated background messages when no Team Run is active
• cover persistence, authentication, CSRF, ownership, validation, and background-message behavior

Closes #3566

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace -- -D warnings
• cargo test --workspace

[Inference1]

@piorpua Would you please review this pr?