feat: implement dynamic fee estimation using horizon base fee checks

[ugoocreates-pixel]

Closes #1823

This PR implements dynamic fee estimation using Horizon base fee checks to ensure system stability, security, and developer ergonomics across the Mobile Money ↔ Stellar Bridge.

Changes

• Replaced the static fetchBaseFee() with the feeStats() endpoint to utilize dynamic fee estimation based on real-time network conditions.
• Integrated fee_charged.p90 as a dynamic base fee (with a fallback to last_ledger_base_fee or 100 stroops) to gracefully handle network congestion and avoid transaction failures.
• Updated the base fee mechanisms across sep31FeeBumpJob, stellarService, and transactions.
• Updated mock implementations and unit tests in transactions.test.ts to support and validate the new dynamic fee estimation logic.

All tests and linting have passed locally.

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