Add migration support to handle schema changes without data loss. Instead of dropping and recreating tables, the system now:
- Uses CREATE TABLE IF NOT EXISTS for initial table creation
- Adds _apply_migrations method to handle incremental schema updates
- Adds _get_table_columns helper to inspect existing table structure
- Adds new columns to model_update_versions table (sort_index, name, base_model, released_at, size_bytes, preview_url, is_in_library, should_ignore)
- Adds should_ignore_model column to model_update_status table
This ensures existing databases are upgraded gracefully while preserving user data.
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