feat(indexer): add configurable automatic event retention cleanup

[Oyinkans0la12]

Closes #77

Summary

Add configurable automatic cleanup of old indexer events to prevent unbounded database growth.

Changes

• Added EVENT_RETENTION_DAYS configuration (default: 0, disabled)
• Added cleanupOldEvents() for deleting expired events
• Runs cleanup once on startup and every 24 hours when enabled
• Deletes records in batches of 1000
• Logs the number of events removed during each cleanup
• Added/updated tests covering cleanup behavior

Configuration

Variable	Default	Behavior
EVENT_RETENTION_DAYS	0	Cleanup disabled
EVENT_RETENTION_DAYS > 0	Enabled	Deletes events older than the configured retention period

Cleanup Behavior

When enabled:

• Runs once during application startup
• Runs every 24 hours thereafter
• Deletes old events in batches of 1000
• Logs the number of deleted events

Testing

Covered:

• Cleanup disabled by default
• Cleanup enabled
• Retention cutoff calculation
• Multi-batch deletion
• Empty cleanup runs
• Cleanup logging

Scope

This PR intentionally focuses only on configurable event retention cleanup. It does not redesign indexing, event persistence, scheduling infrastructure, or database architecture.

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[therealjhay]

Kindly resolve conflict

[Oyinkans0la12]

Kindly resolve conflict

Done.