Fix Grafana crash: revert forced Loki datasource uid

Pinning uid: loki on the already-provisioned Loki datasource made provisioning fail at startup ("Datasource provisioning error: data source not found"), crash-looping the whole Grafana container. Revert the uid pin and instead resolve the datasource in the dashboard via a standard datasource template variable, which works regardless of the datasource's actual (auto-generated) uid.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com Claude-Session: https://claude.ai/code/session_016w3RRcouvZHeMdzRq215KA