fix: resolve bridge and backend issue regressions

[OtowoSamuel]

Summary

• reduce bridge epoch/provider TTL churn
• align sanitization envelopes with the standard ErrorEnvelope contract
• validate Deposit/withdraw amount precision and overflow bounds
• remove plaintext webhook secrets and verify via hash-only comparisons

Changes

• optimize bridge epoch/provider storage and TTL strategy in contracts/bridge-compat/src/lib.rs
• standardize sanitization error responses in backend/src/sanitization.ts
• add Decimal-based money validation and base-unit conversion in backend/src/feeCalculator.ts and backend/src/transferOrchestrator.ts
• hash-only webhook secret handling in backend/src/webhookDelivery.ts and backend/prisma/schema.prisma
• backfill migration and targeted regression tests

Closes #1539
Closes #1554
Closes #1549
Closes #1548

[drips-wave]

@OtowoSamuel Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits