Implements Spec 087: Legacy Runs Removal (rigorous).
### What changed
- Canonicalized run history: **`operation_runs` is the only run system** for inventory sync, Entra group sync, backup schedule execution/retention/purge.
- Removed legacy UI surfaces (Filament Resources / relation managers) for legacy run models.
- Legacy run URLs now return **404** (no redirects), with RBAC semantics preserved (404 vs 403 as specified).
- Canonicalized affected `operation_runs.type` values (dotted → underscore) via migration.
- Drift + inventory references now point to canonical operation runs; includes backfills and then drops legacy FK columns.
- Drops legacy run tables after cutover.
- Added regression guards to prevent reintroducing legacy run tokens or “backfilling” canonical runs from legacy tables.
### Migrations
- `2026_02_12_000001..000006_*` canonicalize types, add/backfill operation_run_id references, drop legacy columns, and drop legacy run tables.
### Tests
Focused pack for this spec passed:
- `tests/Feature/Guards/NoLegacyRunsTest.php`
- `tests/Feature/Guards/NoLegacyRunBackfillTest.php`
- `tests/Feature/Operations/LegacyRunRoutesNotFoundTest.php`
- `tests/Feature/Monitoring/MonitoringOperationsTest.php`
- `tests/Feature/Jobs/RunInventorySyncJobTest.php`
### Notes / impact
- Destructive cleanup is handled via migrations (drops legacy tables) after code cutover; deploy should run migrations in the same release.
Co-authored-by: Ahmed Darrazi <ahmed.darrazi@live.de>
Reviewed-on: #106
