feat(storage): implement protocol-wide Soroban TTL management

[lynaDev2]

Summary

This PR introduces a protocol-wide storage lifecycle system to ensure long-lived protocol state remains available by implementing shared Soroban TTL management across contracts.

Changes

• Added shared TTL constants and storage policy categories for instance, vault, user, reward, lending, and borrowing state.
• Implemented reusable storage helpers to safely extend TTL when records are created, read, or updated.
• Ensured vault metadata and balances persist for the full lock period with an additional safety buffer.
• Added automatic TTL renewal for streak, reward, lending, and borrowing records during normal protocol activity.
• Added handling for records approaching expiry with a consistent renewal/error path.
• Prevented near-expiry or expired state from producing incorrect balances, debt calculations, or reward eligibility.
• Updated affected contracts to use the shared storage helpers instead of direct persistent storage access.
• Added unit tests covering TTL extension behaviour across all affected contracts.
• Added an integration test covering long-lived vaults, streak updates, reward claims, lending positions, and borrowing positions across simulated ledger progression.
• Documented the protocol TTL policy, operational assumptions, and maintenance expectations in the contract README.

Testing

• Verified TTL is extended when records are created, read, and modified.
• Confirmed vault state survives beyond the configured lock period with the required safety buffer.
• Verified streak, reward, lending, and borrowing records renew correctly during normal operations.
• Tested near-expiry and expired record handling.
• Ran unit and integration tests covering long-term ledger progression and state persistence.

Closes #87