feat(transactions): add blockchain recovery and confirmation tracking

[TochukwuJustice]

closes #223

Summary

• Add durable transaction tracking for Stellar and Soroban operations.
• Support pending, successful, failed, and expired lifecycle states.
• Add Horizon and Soroban RPC confirmation polling.
• Add bounded retries with exponential backoff and jitter.
• Prevent duplicate records and conflicting transaction-hash replays.
• Synchronize confirmed contract and milestone state atomically.
• Record transaction lifecycle audit events.
• Add authenticated submission, protected polling, and structured API errors.
• Add a background recovery worker and focused test coverage.

Reason

Blockchain transactions must not update backend state before receiving on-chain confirmation. This change provides durable tracking, retry handling, atomic synchronization, and clear status reporting for transactions that confirm, fail, or expire.

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