feat: add Soroban RPC health check at gateway startup (Closes #7)

[ZacLou]

Closes#7

Changes

Adds startup dependency checks for Soroban RPC connectivity and contract configuration:

• validate_gateway_config(): Expanded to log warnings for missing ESCROW_CONTRACT_ID, GATEWAY_SECRET_KEY, and empty SOROBAN_RPC_URL
• check_soroban_rpc_health(): New async function that probes {SOROBAN_RPC_URL}/health at startup
  • Logs success when RPC responds with 2xx
  • Logs warning when response is non-2xx or connection fails
  • Non-fatal: allows startup even when RPC is temporarily unavailable
  • Skipped entirely when SOROBAN_RPC_URL is not configured
• main.rs: Wired check_soroban_rpc_health into startup sequence after config validation

Files changed

• crates/gateway/src/startup.rs — Expanded config checks + new RPC health probe
• crates/gateway/src/main.rs — Import and call the new health check