feat(monitoring): Display version and build timestamp in Grafana Self-monitoring dashboard

Summary

• Add /version endpoint to Flask backend that reads /VERSION and /BUILD_TS files from the container
• Add version banner panel to Self-monitoring dashboard using the Infinity datasource
• Displays as: PostgresAI v{version} (built: {build_ts}) at the top of the dashboard

Changes

File	Change
monitoring_flask_backend/app.py	Added /version endpoint returning {"version": "...", "build_ts": "..."}
config/grafana/dashboards/Self_Monitoring_Dashboard.json	Added full-width stat panel querying the version endpoint

Test plan

• Deploy the stack and verify the version banner appears at the top of the Self-monitoring dashboard
• Verify /version endpoint returns correct JSON when VERSION/BUILD_TS files exist
• Verify graceful fallback to "unknown" when files don't exist

https://gitlab.com/postgres-ai/platform/-/issues/346