Add database migration for job progress columns

[alokemajumder]

Summary

Adds Alembic migration for the new job progress tracking columns added in PR #34.

New Columns

Column	Type	Description
current_stage	String	Alias for stage (worker compatibility)
status_message	String	Progress status message
updated_at	DateTime	Last update timestamp
processing_stats	JSON	Detailed processing statistics

Migration

# Run migration
alembic upgrade head

# Rollback if needed
alembic downgrade 003

Test plan

• Run migration on test database
• Verify columns are created
• Test rollback works correctly