feat(keeper): add retry logic for failed charge attempts

[Demilade10]

Description

Add retry logic to the keeper script so transient failures (network errors, ledger congestion, temporary RPC issues) do not silently drop a scheduled charge. Currently a failed charge() call has no retry mechanism.

Requirements and Context

• keeper.js currently calls charge() on due subscriptions but does not distinguish between a permanent failure (insufficient allowance, cancelled subscription) and a transient one (RPC timeout, network blip)
• Retries must not double-charge a subscriber - the keeper must confirm a charge did not succeed before retrying
• Uses @stellar/stellar-sdk for all chain interaction

Suggested Execution

Branch name: feat/keeper-retry-logic

Files to touch:

• keeper/keeper.js
• keeper/retry.js (new - retry/backoff helper)

Example commit message: feat(keeper): add retry logic with exponential backoff for failed charges

Test and Commit

• Distinguishes transient failures (retryable) from permanent failures (not retryable, e.g. Unauthorized, insufficient allowance)
• Implements exponential backoff with a max retry cap (e.g. 3 attempts)
• Confirms via transaction status lookup that a charge did not succeed before retrying, to prevent double-charging
• Logs each retry attempt with reason and outcome
• Permanent failures are logged and skipped, not retried indefinitely
• Include a test run log in the PR showing a simulated transient failure recovering on retry

Guidelines

• Assignment required before starting - comment to request assignment
• PR description must include: Closes docs(dev): add local development setup guide #5

Complexity

High - 200 points