Add 'om migrate stamp' command to mark migrations as applied

[abrookins]

Summary

Add om migrate stamp command to mark migrations as applied without actually running them.

Use Cases

1. Importing existing schema: When adopting Redis OM on an existing database where indexes already exist
2. Manual migrations: When you've manually applied schema changes and need to update tracking
3. CI/CD workflows: Mark specific migration states for testing or deployment

Proposed Interface

# Mark all pending migrations as applied
om migrate stamp --all

# Mark specific migration as applied
om migrate stamp <migration_id>

# Mark up to a specific migration
om migrate stamp --target <migration_id>

Implementation Notes

• Add stamp method to SchemaMigrator that calls mark_applied() without running up()
• Add CLI command with appropriate options
• Similar to Alembic's stamp command