feat(listener): add configurable event retention policy (#626)

[Abdullahi-Code9]

Summary

Resolves #626 by making processed event metadata retention configurable (PROCESSED_EVENT_RETENTION_MS) and integrating safe periodic cleanup of expired processed_events records without interrupting active deduplication or cursor tracking.

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Key Changes

• Configurable Retention Duration: Introduced PROCESSED_EVENT_RETENTION_MS environment variable defaulting to 30 days (2592000000 ms).
• Validation Guards: Added validation in validateConfig rejecting non-integers, non-numeric strings, and durations under 60 seconds (60000 ms).
• Safe Expired Record Removal: Updated CleanupService to run cleanupOldEvents using SQLite UTC relative datetime offsets while leaving event_cursors intact.
• Documentation & Unit Tests: Added coverage for configuration boundaries and cleanup invocation in config.test.ts and cleanup-service.test.ts, and updated .env.example and ENVIRONMENT_VARIABLES_AND_SECRETS.md.

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Acceptance Criteria Checklist

• Retention duration is configurable via PROCESSED_EVENT_RETENTION_MS.
• Expired processed_events records are safely removed based on configured age.
• Active deduplication and block cursor state remain preserved.
• Invalid values (non-integers, < 60000 ms) are rejected during configuration loading.

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