feat: expose gateway signer XLM balance as Prometheus gauge (Closes #29)

[ZacLou]

Closes #29

Changes

Adds waveflow_gateway_signer_xlm_balance Prometheus gauge that queries the gateway signer's XLM balance from Horizon:

• Config: Added HORIZON_URL and GATEWAY_PUBLIC_KEY env vars
• Gauge function: update_signer_xlm_gauge() queries Horizon /accounts/{pubkey} and sets the gauge
• Periodic update: Background tokio task runs every 5 minutes
• Graceful degradation: Skips when GATEWAY_PUBLIC_KEY or HORIZON_URL not configured

Files changed

• crates/shared/src/config.rs — Added horizon_url and gateway_public_key fields
• crates/gateway/src/attestation.rs — Added update_signer_xlm_gauge() function
• crates/gateway/src/main.rs — Spawns periodic gauge update task

Alerting use case

Monitor waveflow_gateway_signer_xlm_balance for low balance → the signer needs XLM to submit Soroban transactions.