Add logs retention

[mishankov]

Closes #21

Summary

Adds a retention parameter to config that enables automatic cleanup of old request logs from the database.

Changes

• Config (internal/config/config.go): Added Retention field (int, seconds)
• Service (internal/requestlog/service.go): Added DeleteOlderThan(thresholdUnix int64) method
• App (internal/app/build.go):
  • Starts retention scheduler when retention > 0
  • Cleanup runs every hour (configurable via cleanupInterval constant)
  • Proper context cancellation for graceful shutdown
  • Scheduler registered as startup task
• Tests (internal/requestlog/handler_test.go): Added test for DeleteOlderThan functionality

Behavior

• If retention is not set or 0, no cleanup occurs (existing behavior)
• If retention is set to N seconds, logs older than N seconds are deleted every hour
• Cleanup runs as a background goroutine with proper lifecycle management
• Errors during cleanup are logged but do not crash the application

Testing

• All existing tests pass
• New test TestDeleteOlderThan_RemovesOnlyOldLogs verifies correct deletion behavior
• Build succeeds with race detector enabled

Code Review

Addressed P2 findings from code review:

• Added context cancellation for graceful shutdown
• Changed cleanup interval to fixed 1 hour instead of retention period (to avoid logs living up to 2x retention)