feat(agent): wire rebalance execution to vault contract

[edehvictor]

Closes #33

Changes

• replace mock rebalance transaction generation in src/agent/router.ts with a real call to the vault Soroban contract
• convert best APY percent values into expectedApyBasisPoints (u32) before invoking the contract
• persist the returned on-chain txHash as a Prisma Transaction with:
  • type: REBALANCE
  • status: PENDING
• keep rebalance details returned from the agent flow aligned with the real contract submission result
• start the vault event listener during app startup so rebalance confirmations can be picked up by the existing event-processing path
• add unit coverage that mocks Stellar contract calls and verifies:
  • APY to basis-point conversion
  • real tx hash persistence
  • pending rebalance transaction creation

Notes

• Rebalance execution now submits a real agent-signed Soroban transaction through src/stellar/contract.ts
• Transaction confirmation and follow-up DB updates remain delegated to the vault event listener flow

Testing

• node .\node_modules\typescript\bin\tsc --noEmit
• node .\node_modules\jest\bin\jest.js --runInBand

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