Atlas sync now distinguishes between hosts Atlas actively tracks (returned
plans, active or inactive) vs hosts with empty responses (not in Atlas).
Only atlas_known hosts show the badge — ACCESS-OPS hosts not covered by
Atlas won't show the amber '0' warning badge anymore.
Changes:
- Migration adds atlas_known BOOLEAN column to atlas_action_plans_cache
- Sync sets atlas_known = true only when Atlas returns at least one plan
- Metrics endpoint only counts atlas_known hosts in its aggregation
- Status endpoint includes atlas_known in response
- AtlasBadge renders nothing when atlas_known = false
- Bulk-create and refresh-cache upserts set atlas_known = true
- Backfill marks existing hosts with plans + managed BU hosts as known
