feat(yearn): use $200k large-flow threshold on L2s

[spalen0]

Summary

• Base, Arbitrum and Katana large-flow alerts now fire at >= $200k for both deposits and withdrawals.
• Replaces the Katana-only $50k withdrawal override (KATANA_WITHDRAWAL_THRESHOLD_USD) with L2_CHAIN_IDS + L2_THRESHOLD_USD.
• Mainnet unchanged at the $500k --threshold-usd default; that flag now only affects mainnet.
• Updated monitoring.yaml and protocols/yearn/README.md.

Chain	Before (dep / wd)	After (dep / wd)
Mainnet	$500k / $500k	$500k / $500k
Base	$500k / $500k	$200k / $200k
Arbitrum	$500k / $500k	$200k / $200k
Katana	$500k / $50k	$200k / $200k

Test plan

• ruff format / ruff check
• pytest tests/test_monitoring_config.py

🤖 Generated with Claude Code