BE-49: Testnet Data Reset + Reindex Tooling (Safe, Repeatable)

[Cedarich]

Complexity: 150 points
Branch: feat/be-testnet-reset-reindex
Summary
Provide tooling to reset testnet data and reindex Soroban events from a known checkpoint so demos and contributor testing remain reproducible.
Tasks

• Add an admin-only endpoint or job to wipe testnet-only tables safely.
• Reinitialize checkpoints and rerun indexing over a configured ledger range.
• Add safeguards to prevent running reset outside testnet environment.
• Add audit logs and metrics for reset/reindex operations.
  Acceptance Criteria
• Reset can only run on testnet and is blocked otherwise.
• Reindex produces deterministic state for the configured ledger range.
• Operators can validate completion and resulting counts quickly.

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