fix: wire Soroban service to live RPC

[Ibinola]

Summary

• Replace the Soroban mock path with a live RPC-backed client when CONTRACT_ID_GIST_REGISTRY is configured
• Submit post_gist transactions on-chain and decode get_gist results from Soroban RPC
• Keep mock behavior intact when no contract ID is configured
• Fix the indexer cursor to advance by Soroban ledger, not timestamp

Validation

• npm run build
• npm test -- --runInBand --watchman=false gists.service.spec.ts indexer.service.spec.ts

Closes #603