feat(postgres): infrastructure setup and schema creation (tasks 1-2)
- Install pg (node-postgres) dependency
- Create backend/db.js connection pool module (max 10, auto-reconnect)
- Install Docker and spin up steam-postgres container on port 5433
- Create backend/db-schema.sql with complete Postgres DDL (24 tables)
- Replace findings_json blob with ivanti_findings table (individual rows)
- Merge notes/overrides into findings table columns
- Add proper indexes: state, bu_ownership, severity, composite
- Create backend/setup-postgres.js for idempotent schema initialization
- Add DATABASE_URL to .env and .env.example
- Update migration plan docs with Docker setup commands
- Verify: schema executes cleanly, pool connects, 24 tables created
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