Lease Coordination Worker for Automated Initialization (#13)

[kryputh]

This PR implements the coordination worker required for issue #13.

Key Features:

• Backend worker to monitor signing states.
• API endpoint /sign-lease to collect landlord and tenant signatures.
• Automated triggering of initialize_lease on Soroban when BOTH parties have signed.
• Prevention of "Ghost Leases" by ensuring on-chain state only exists when legal signatures are collected.

Tests:

• Unit tests for coordination logic (waiting for multiple signatures).
• API integration check for project status.

Closes #13

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