feat(metering): build subscription metering with real-time usage tracking

[morelucks]

Closes #963

Summary of Changes

1. Smart Contract ()

• Implemented on Soroban Rust SDK.
• Support for metered plans, usage tracking per subscription, accrued fee calculation (base price + excess units * unit rate), hard usage cap limits, period resets, usage history logging, and rate limits.
• Full Soroban unit test suite covering critical paths, authorization, limit enforcement, and period resets.

2. Frontend Zustand Store ( & )

• Implemented with and persistence.
• Features: real-time usage recording, cost accrual calculations, spending limits enforcement, threshold alert triggers (80%, 90%, 100%), cycle resets, alert acknowledgement, and telemetry simulation.
• Exported via and .
• Comprehensive Jest unit test coverage in and .

3. Documentation & Benchmarks ()

• Detailed contract API specification, frontend store usage examples, telemetry setup guide, and performance benchmark tables.

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[Smartdevs17]

Thanks for contributing! The changes have been merged. Feel free to leave a review or feedback.