[api] Implement POST /transactions Soroban transaction relay endpoint

[Mimah97]

Problem

The frontend swap, remove-liquidity, and fee-collection flows POST signed XDR to ${API_BASE}/transactions, but no controller implements this route. Swaps cannot complete end-to-end on testnet.

Affected callers:

• apps/web/hooks/useSwapExecution.ts
• apps/web/hooks/useRemoveLiquidity.ts
• apps/web/app/portfolio/page.tsx (fee collection)

Acceptance criteria

• Add TransactionsController with POST /transactions accepting { xdr: string }
• Submit transaction via @stellar/stellar-sdk RPC using STELLAR_RPC_URL
• Return { hash: string } on success
• Map Soroban slippage errors to { code: "SLIPPAGE_EXCEEDED" }
• Unit + e2e tests with mocked RPC

Files

• New module under apps/api/src/transactions/