feat(backend): improve transaction reliability and migration safety

[eyooTech]

Summary

This PR introduces backend reliability and operational-safety improvements across payout history backfills, database migration validation, and RPC-backed read caching.

Changes

• Added a payout history backfill dry-run mode that allows operators to preview:

  • Ledger ranges being processed
  • Events discovered
  • Existing records
  • Records that would be created
  • Sample records
  • Without making any database writes

• Added database migration checksum verification to detect modified or missing applied migrations before the application starts serving traffic.

  • Production fails fast when migration checksums differ.
  • Non-production environments can report mismatches as warnings.
  • Added documentation covering detection and recovery procedures.

• Added stale-if-error behavior for eligible read-cache entries during transient RPC failures.

  • Stale data is only served when explicitly enabled for an endpoint.
  • Cached values are subject to a maximum stale age.
  • Responses/logs expose stale-state metadata.
  • Added failure and RPC recovery test coverage.

Testing

• Added unit tests for backfill dry-run behavior and verified that no database writes occur during preview mode.
• Added migration checksum tests for matching, modified, missing, and production-failure scenarios.
• Added stale-cache tests covering fresh reads, RPC failures, expired cache entries, disabled fallback, and RPC recovery.

Issues

Closes #1036
Closes #1035
Closes #1034
Closes #1037

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