feat: escrow low-balance alerting with Prometheus counter (Closes #45)

[ZacLou]

Closes #45

Changes

Implements escrow low-balance alerting per PRD Section 8:

• Config: Added ESCROW_LOW_BALANCE_THRESHOLD env var (default 0 = disabled)
• Alert function: check_escrow_balance_alert() in attestation.rs checks program escrow_balance after each payout
• Prometheus counter: waveflow_gateway_escrow_low_balance_total increments when balance drops below threshold
• Logging: tracing::warn! with program_id, repo, balance, and threshold fields
• Integration: Wired into process_merge() in routes.rs, called after increment_milestone_spent()
• .env.example: Added documentation for the new threshold variable

Files changed

• crates/shared/src/config.rs — Added escrow_low_balance_threshold field
• crates/gateway/src/attestation.rs — Added check_escrow_balance_alert() function
• crates/gateway/src/routes.rs — Wired alert into payout processing pipeline
• .env.example — Documented ESCROW_LOW_BALANCE_THRESHOLD