feat: add USTB (Superstate) monitoring

[spalen0]

Summary

• Adds monitoring for Superstate USTB (Short Duration US Government Securities Fund) on Ethereum Mainnet
• New ustb/ folder with hourly monitoring script covering NAV/Share, RedemptionIdle, supply changes, and oracle staleness
• Adds USTB to stables/main.py for DeFiLlama price checking
• Wired into hourly GitHub Actions workflow

Monitoring checks

Check	Severity	Threshold
NAV/Share decrease (checkpoint comparison)	CRITICAL	Any decrease
Oracle divergence (Continuous vs Chainlink)	HIGH	> 0.5%
RedemptionIdle USDC balance	HIGH	< $500K
Supply change (24h cached baseline)	HIGH	> ±10%
Oracle staleness	HIGH	> 4 days (reverts at 5)
DeFiLlama price (via stables)	CRITICAL	< $10.50

Implementation details

• Uses 2 batched RPC calls (6 calls in batch 1, 2 in batch 2) instead of individual calls
• 24h supply baseline uses cache-id.txt (shared hourly workflow cache) with separate keys for supply value and timestamp
• No events used — all monitoring is via contract reads

Closes #189

Test plan

• Verify RPC calls return expected data with a manual python ustb/main.py run
• Confirm alerts route to correct Telegram channel
• Validate cache initialization on first run (no false alerts)
• Check that stables monitoring picks up USTB price

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