feat: estimate transaction fees before Soroban submission

[samuelisi]

Summary

• Adds StellarTxService (src/soroban/stellar-tx.service.ts) with estimateFee and prepareTransaction, which use SorobanRpc.Server's simulateTransaction + getFeeStats APIs to compute base fee + resource fee instead of hardcoding a fee value.
• Base (inclusion) fee is congestion-aware: sourced from network fee stats at a configurable percentile (stellar.feePercentile, env STELLAR_FEE_PERCENTILE, defaults to p50), falling back to BASE_FEE if fee stats are unavailable or degenerate.
• Extends SorobanService with thin wrappers (getFeeStats, simulateTransaction, prepareTransaction) around the previously-private SorobanRpc.Server instance so this logic is testable without hitting the network.
• Registers StellarTxService in SorobanModule so it's available for the settlement transaction flows landing in later issues.

Closes #31
closes #25
closes #35
closes #32

Test plan

• npm run lint
• npm run typecheck
• npm test (24 passed, including 6 new specs in stellar-tx.service.spec.ts covering base-fee percentile lookup, fallback to BASE_FEE on RPC failure/degenerate stats, fee-estimate arithmetic, simulation-error handling, and prepared-transaction fee application)
• npm run test:e2e (23 passed)

Full command output

$ npm run lint

> vortex-backend@0.1.0 lint
> eslint src test scripts --ext .ts


$ npm run typecheck

> vortex-backend@0.1.0 typecheck
> tsc --noEmit


$ npm test

> vortex-backend@0.1.0 test
> jest

Test Suites: 4 passed, 4 total
Tests:       24 passed, 24 total
Snapshots:   0 total
Time:        4.387 s
Ran all test suites.

$ npm run test:e2e

> vortex-backend@0.1.0 test:e2e
> jest --config ./test/jest-e2e.json

Test Suites: 5 passed, 5 total
Tests:       23 passed, 23 total
Snapshots:   0 total
Time:        6.217 s, estimated 10 s
Ran all test suites.

🤖 Generated with Claude Code

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