Feature Request: Dashboard templates served directly from filesystem without database storage for multi-tenant deployments

### Why is this needed? In large-scale multi-tenant SaaS deployments, Grafana instances can host thousands of organizations (10,000+), each representing an isolated customer. Many of these organizations require the same standard dashboards (system metrics, application monitoring, alerts, etc.) with identical JSON definitions. Currently, Grafana stores a complete copy of each dashboard in the database for every organization, leading to massive data duplication: Example scenario: 10,000 organizations 10 standard dashboards per organization Result: 100,000 duplicated dashboard records in the database This problem is significantly amplified by dashboard versioning: Default versions_to_keep = 20 With provisioning or periodic updates, version history grows rapidly Real-world reports show dashboard_version tables with thousands of duplicate entries (see [#13043](https://github.com/grafana/grafana/issues/13043)) Users have reported grafana.db files reaching 15-17GB, primarily due to version history Additionally, disabling versioning is not viable because organizations also have unique, custom dashboards that require version history. ### What would you like to be added? A template dashboard mode that serves dashboards directly from the filesystem to all organizations without storing anything in the database. ### Proposed architecture: Template Dashboards on Filesystem Dashboard JSON files stored on the filesystem (e.g., /var/lib/grafana/dashboards/templates/) Versioned in Git as part of "Observability as Code" workflows Not stored in the database at all No Database Storage No references, no copies, no version history in the database Dashboards are read directly from filesystem when accessed Zero database footprint for template dashboards Visibility and Behavior Template dashboards are visible to all organizations Appear in a dedicated folder (e.g., "Standard Dashboards" or "Templates") Completely read-only - cannot be edited, saved, or deleted from the UI No version history …