feat(listener): make blockchain event batch size configurable (#627)

[Abdullahi-Code9]

Summary

Resolves #627 by adding operator configuration for the number of blockchain events fetched per polling cycle (EVENT_BATCH_SIZE), replacing hardcoded event query limits while maintaining backwards compatibility and default behavior.

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Key Changes

• Configurable Batch Size: Introduced EVENT_BATCH_SIZE environment variable defaulting to 100.
• Input Validation: Added validation in validateConfig to ensure eventBatchSize is a positive integer (>= 1), rejecting invalid/negative values.
• RPC Query Update: Configured EventSubscriber to pass config.eventBatchSize to Stellar RPC getEvents requests.
• Documentation & Testing: Added unit tests in config.test.ts and event-subscriber.test.ts, and documented the new variable in .env.example and ENVIRONMENT_VARIABLES_AND_SECRETS.md.

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Acceptance Criteria Checklist

• Batch size is configurable via EVENT_BATCH_SIZE.
• Documented default (100) is provided in .env.example and documentation.
• Invalid values (0, negative numbers, non-integers) are rejected during config validation.
• Existing polling behavior remains unchanged when using the default batch size.

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