Add source status API and enhance dashboard with data source checks

- Introduced a new API endpoint `/api/source-status/` to return the status of Prometheus and OpenStack data sources.
- Implemented lightweight health check functions for both Prometheus and OpenStack.
- Updated the dashboard template to display the status of data sources dynamically.
- Added tests for the new API endpoint to ensure correct functionality and response handling.
- Configured a cache timeout for source status checks to improve performance.

watcher_visio/settings.py

@@ -155,3 +155,4 @@ CACHES = {
     }
 }
 DASHBOARD_CACHE_TTL = 120  # seconds
+SOURCE_STATUS_CACHE_TTL = 30  # seconds (lightweight source-status checks)