Add compound XDR builder endpoint

[YfengJ]

Summary

• Add a TransactionsModule with POST /api/v1/transactions/build for building unsigned Stellar transaction envelopes.
• Validate request shape, source/destination public keys, asset metadata, amounts, optional fee, operation count, and timeout.
• Load the source account sequence through Horizon and use TransactionBuilder to assemble sequential payment and pathPaymentStrictSend operations into one base64 XDR response.

Closes #155

Validation

• npm ci --legacy-peer-deps --ignore-scripts --no-audit --no-fund
• npx jest src/transactions/transactions.service.spec.ts src/transactions/transactions.controller.spec.ts --runInBand ✅ 2 suites / 3 tests passed
• npm test -- --runInBand ❌ existing unrelated failure in src/pools/pools.controller.int.spec.ts where res.body.status is undefined instead of success
• npm run build ❌ existing unrelated compile failures around logger LogLevel, missing dynamic module imports, Prisma generated members, express-rate-limit, and websocket/prisma model types; no errors were reported from the new transactions files
• git diff --check ✅

[AlAfiz]

@YfengJ please work on the CI build it failed

[YfengJ]

Thanks for the heads-up. I pushed a CI fix in 16ca24e.

What changed:

• aligned the Nest websocket packages so npm ci no longer hits the reflect-metadata peer dependency conflict
• fixed the TypeScript build errors from logger/websocket/invoice imports
• wrapped the APY history response in the success envelope expected by the integration test

Verified locally with:

• npm ci --no-audit --no-fund
• npm run build
• npm test
• git diff --check

[YfengJ]

Follow-up: upstream main moved after my CI fix, so I merged the latest main into this branch and resolved the app module conflict without dropping the transaction, orders, or gas modules.

Latest head is 2511472. I re-ran the CI-equivalent local checks successfully:

• npm ci --no-audit --no-fund
• npm run build
• npm test
• git diff --check

[YfengJ]

One more note: the latest API CI run for head 2511472 appears to be in action_required rather than failed: https://github.com/BETAIL-BOYS/TradeFlow-API/actions/runs/28102816366

It looks like it may need maintainer approval to run for the forked PR. I do not see a new code failure from this latest run yet.