feat(diagnostics): add lightweight validation endpoints for analytics…

[Confidence001Jewel-cmd]

Close #645

PR Description

• Summary: Add lightweight diagnostics endpoints to enable local end-to-end validation of analytics and cost tracking and provide a minimal Grafana dashboard import for quick verification.
• Changes: Added validation endpoints and registered them in the app module; imported a minimal Grafana dashboard for TeachLink Overview.
  • Files: validation.controller.ts, app.module.ts
• Behavior:
  • POST /analytics/event — accepts {category, action, label?, value?} and increments feature_events_total.
  • POST /metrics/cost — accepts {amountUsd} and sets infrastructure_hourly_cost_usd.
  • GET /metrics — exposes Prometheus-format metrics for scraping.
• Why: Facilitates fast local validation of analytics → Prometheus → Grafana without requiring the app Docker image build; useful for CI/debugging and onboarding.
• Testing performed (local):

npm install
npm run start:dev
cd infra/monitoring
docker compose up -d prometheus grafana
# Send events
curl -X POST http://localhost:3000/analytics/event -H 'Content-Type: application/json' -d '{"category":"feature","action":"launch_button_clicked"}'
curl -X POST http://localhost:3000/metrics/cost -H 'Content-Type: application/json' -d '{"amountUsd": 5}'
# Verify Prometheus
http://localhost:9090/api/v1/query?query=feature_events_total
http://localhost:9090/api/v1/query?query=infrastructure_hourly_cost_usd
# Grafana UI
http://localhost:3001 (admin/admin)
.

[RUKAYAT-CODER]

kindly resolve conflict and fix wokflow