[gateway] Escrow low-balance alerting (PRD Section 8)

[yinkscss]

Context

PRD v0.2 Operational Requirements require escrow low-balance monitoring and high-severity alerts on InsufficientEscrow.

Problem

Contract reverts on insufficient escrow but maintainers are not proactively alerted before depletion.

Proposed scope

• Periodic check of on-chain escrow balance vs configurable threshold
• Prometheus gauge/alert for low escrow per program
• Increment high-severity metric on InsufficientEscrow chain errors ([gateway] Surface InsufficientEscrow chain errors with alert metrics #21 related)

Acceptance criteria

• Configurable low-balance threshold per program or global env
• Metric exposed for alerting (e.g. waveflow_escrow_balance_low)
• Documented in maintainer runbook ([documentation] Add Render production deployment runbook #39)

References

• docs/PRD.md Section 8
• crates/gateway/src/attestation.rs
• contracts/waveflow-escrow/src/contract.rs