Add dashboard/panel/scope-link tables, models, and seed

Three tables managed independently of the metric catalog:
- dashboard {id, title, metadata}
- panel {id, dashboard_id (FK), title, prometheus_query_preset_id, rank} - dashboard 1:N panel
- scope_dashboard_link {scope_type, dashboard_id}

Repository provides get_dashboard_for_scope(scope_type) returning the linked dashboard + rank-ordered panels. Seed a default dashboard + panels + link.

Panel query/display attributes (mode, unit) are owned by the referenced preset; viz is out of scope for now. One dashboard per scope_type (scope_type is the link PK). Panel FK cascades on dashboard delete.

Success Criteria:
- [ ] Migration creates dashboard/panel/link tables with the 1:N and link relations
- [ ] Seed: a default dashboard with >=1 rank-ordered panel linked to a scope_type
- [ ] get_dashboard_for_scope(scope_type) returns the dashboard with panels in rank order
- [ ] Deleting a dashboard cascades panels
- [ ] pants test passes for affected packages

JIRA Issue: BA-6751

Start by locating the repository implementation of get_dashboard_for_scope(scope_type) and the migration and seed conventions in the affected packages. Add the dashboard, panel, and scope_dashboard_link tables, default linked data, rank-ordered retrieval, and cascade behavior, then run pants test for the affected packages.

Written by the indexing model from the issue text.