feat: add indexer lag guard to prevent broken UX during testnet insta…

[fadee26]

Indexer Lag Guard

Prevent broken UX during testnet instability by detecting Soroban indexer lag and automatically disabling risky write flows when the system is too far behind.

Features:

• Define "lag" based on latest indexed ledger vs current network ledger with configurable thresholds (via INDEXER_LAG_THRESHOLD_LEDGERS env var, default: 100)
• Add decorator @RequiresIndexerLagCheck() to mark endpoints that should be blocked when lag is too high
• Add global guard that blocks selected endpoints when lag exceeds threshold
• Surface stable error code INDEXER_LAGGING telling clients to retry later
• Add metrics (indexer lag in ledgers, blocked requests, guard status) and audit logs for blocked requests
• Add admin override (via INDEXER_LAG_GUARD_OVERRIDE=true env var)
• Read‑only endpoints remain available

Acceptance Criteria:

✅ When lag exceeds threshold, risky flows are blocked consistently
✅ Read‑only endpoints remain available
✅ Operators can observe lag and overrides via metrics/logs

closes #534

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