feat: [#238] add E2E validation and manual testing guide for Prometheus (Phase 7)

- Add Prometheus configuration file validation to release tests
  - Create PrometheusConfigValidator for remote file verification
  - Validates prometheus.yml exists at /opt/torrust/storage/prometheus/etc/
  - Checks file permissions and ownership
- Refactor validation with ServiceValidation struct for extensibility
  - Replace boolean parameter with flags struct for future services (Grafana, etc.)
  - Supports selective validation based on enabled services
- Update e2e-deployment environment to include Prometheus
  - Add prometheus config with 15s scrape_interval
  - Create e2e-deployment-no-prometheus.json for disabled scenario testing
- Manual E2E testing completed and verified:
  - ✅ Prometheus container running (prom/prometheus:v3.0.1)
  - ✅ Both tracker endpoints (stats & metrics) scraped successfully
  - ✅ Prometheus UI accessible and functional
  - ✅ Metrics collection verified over time
- Add comprehensive manual testing documentation
  - Created docs/e2e-testing/manual/prometheus-verification.md
  - Documents verification steps for container, config, targets, UI, and metrics
  - Includes troubleshooting guide for common issues
  - Provides success criteria checklist
- All linters passing, all E2E tests passing (1507+ tests)

Architecture validated: Independent service rendering pattern working correctly
with Prometheus fully integrated into deployment workflow.

Author: josecelano