Adds the cross-target queries the unified Backups page needs:
- GET /api/v1/backups/policies (filters: target_type, enabled) — replaces the
legacy filesystem schedule list with one view of every BackupPolicy.
- GET /api/v1/backups/runs (filters: target_type, status, policy_id, limit≤500)
— every BackupRun across all policies, each augmented with its target's
type/id/subtype so the UI filters without a second lookup.
Both admin-guarded like the rest of /backups.
Verified: 6 new API tests pass.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
