feat(vault): add vault state and authenticated balance read APIs

[Kaylahray]

✅ Issue complete: Vault Read APIs shipped. closes #25

Hi team, I’ve completed this issue end-to-end.

What I delivered:

• Added on-chain read integration for APY, active protocol, and user balance/shares
• Added new endpoints:
  • GET /api/vault/state
  • GET /api/vault/balance (authenticated, uses the logged-in user wallet)
• Added integration tests with mocked contract reads (no real RPC calls)

Result:

• Response shapes match the spec
• Auth flow is enforced for user balance
• Tests and lint are passing

Ready for review 🚀