Add warning when pending migrations exist

[abrookins]

Summary

Add a warning message when there are pending migrations that haven't been applied.

Current Behavior

om migrate status shows pending count but there's no proactive warning when using other commands or at startup.

Proposed Behavior

1. CLI Warning: When running om migrate status, show a colored warning if migrations are pending:

   ⚠️  Warning: 3 pending migrations not applied. Run 'om migrate run' to apply.
   

2. Optional startup warning (stretch goal): Add a function that applications can call to check migration status and log a warning if behind.

Implementation Notes

• Update status command output to highlight pending migrations
• Consider adding --check flag that exits non-zero if behind (useful for CI)