Adds migration_runner.py that executes pending migrations from
migrations/ directory when backend-compliance starts. Tracks applied
migrations in _migration_history table.
Handles existing databases: detects if tables from migrations 001-045
already exist and seeds the history table accordingly, so only new
migrations (046+) are applied.
Skippable via SKIP_MIGRATIONS=true env var.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
