feat(contracts): proxy-based contract upgradability (#176)

[Edoscoba]

Implements a proxy-pattern upgrade flow for Soroban contracts so the stable contract ID never changes while logic can be upgraded safely.

Key changes:

• Adds UpgradeableProxy contract with timelocked authorize_upgrade, schedule_upgrade, upgrade_to, and rollback flows.
• Introduces SubTrackrStorage contract to preserve state across upgrades; only the active implementation can write.
• Updates SubTrackrSubscription implementation to read/write via the storage contract and adds get_version, validate_upgrade, and migrate handlers.
• Tracks upgrade history and maintains a previous-implementation stack for rollback.
• Adds deployment/upgrade scripts and documents procedures (contracts/DEPLOYMENT.md).
• Adds integration tests covering upgrade, migration, validation failure, and rollback scheduling (contracts/proxy/tests/upgrade_flow.rs).

closes #176

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