feat: implement Soroban-backed billing service

[dimka90]

Summary

• replace the placeholder billing flow with a Soroban-backed billing service that checks balance, retries transient deduct failures, and records usage events only on successful deducts
• add a dedicated Soroban billing RPC client for balance and deduct invocations with optional idempotency passthrough
• update the billing route and environment documentation for the new Soroban billing settings

Verification

• npm test -- --runInBand src/services/billing.test.ts src/services/sorobanBilling.test.ts initially failed because jest was not installed locally
• npm run typecheck initially failed because tsc was not installed locally
• npm install --no-audit --no-fund was started to restore local tooling but did not produce usable binaries within this session, so checks remain unverified here
• per instruction, I did not spend time fixing unrelated repo or environment issues beyond that

Closes #22

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