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

backend/.env.example

@@ -60,6 +60,11 @@ CARD_API_PASS=
 # Set to true if behind Charter's SSL inspection proxy
 CARD_SKIP_TLS=false
 
+# PostgreSQL Database (Docker container steam-postgres)
+# If set, the backend uses Postgres instead of SQLite.
+# Format: postgresql://user:password@host:port/database
+DATABASE_URL=postgresql://steam:<password>@localhost:5433/cve_dashboard
+
 # GitLab Feedback Integration (bug reports and feature requests from the dashboard)
 # PAT needs 'api' scope. Project ID is the numeric ID from GitLab project settings.
 GITLAB_URL=http://steam-gitlab.charterlab.com