feat(scripts): add exponential backoff retry logic and DLQ for keeper (#76)

[Abdullahi-Code9]

Closes #582

📝 Description

This PR introduces robust exponential backoff retry logic and a Dead Letter Queue (DLQ) persistent storage mechanism for the PayFlow keeper service (scripts/keeper.ts), along with a dedicated DLQ replay utility (scripts/replay-dlq.ts).

🚀 Changes Made

1. Exponential Backoff & Smart Error Categorization

• Retry Helper: Wrapped charge execution in retryWithBackoff() with configurable max retries and exponential delay (baseMs * 2^attempt + jitter).
• Short-Circuit Permanent Errors: Detects unrecoverable contract errors (NoSubscriptionFound, SubscriptionInactive, ContractPaused, Unauthorized) and bypasses retries immediately to avoid wasting RPC budget.

2. Dead Letter Queue (failed-batches.jsonl)

• Pushes failed batches to an append-only JSONL file in data/dlq/failed-batches.jsonl upon exhausting all retry attempts.
• Records full metadata: timestamp, offset, limit, error, tx_xdr, and attempts.

3. DLQ Replay CLI (scripts/replay-dlq.ts)

• Reads failed-batches.jsonl line-by-line and attempts re-execution of transient failures.
• Supports --dry-run to preview pending entries without submitting transactions.
• Rotates resolved entries to replayed-batches.jsonl and moves unrecoverable entries to dead-batches.jsonl.

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🧪 Verification & Testing

Type Safety Verification

Verified strict TypeScript types across all scripts:

npx tsc --noEmit