feat(fleet-sync): replica self-demote endpoint and role UX (#969)

A replica admin can now demote the instance back to a standalone
control without raw SQLite access. The Settings → Security UI surfaces
a confirm-gated button when the role is replica; the role probe also
surfaces a soft banner when it cannot determine fleet role rather
than silently defaulting to control.

Backend:
- POST /api/fleet/role/demote (admin, requires `{confirm: true}`):
  flips fleet_role to 'control', clears fleet_self_identity,
  fleet_control_identity, and both received_pushed_at:* watermarks,
  drops every replicated_from_control row from scan_policies and
  cve_suppressions, nulls out any orphaned policy_evaluation cache.
  Returns 409 ALREADY_CONTROL when invoked on a control.
- DatabaseService gains `clearOrphanPolicyEvaluations()` and
  `clearReplicatedRows()` helpers. Reanchor consolidates onto
  clearReplicatedRows so it shares the same code path.
- `FleetSyncService.demote()` returns boolean for the route to
  translate into 200 or 409.

Frontend:
- SecuritySection probes /fleet/role and now records explicit success
  vs failure rather than silently treating an error as control. A
  soft banner appears when probe fails.
- Replica banner gains a "Demote to control" button and a destructive
  ConfirmModal explaining the wipe.

Tests:
- 4 new route-level vitest cases (401, 400 without confirm,
  end-to-end demote with replica setup, 409 ALREADY_CONTROL with
  explicit precondition).
- Service unit test asserts the consolidated clearReplicatedRows path.
- Full backend suite: 1773 pass / 5 skipped. Frontend: 185 pass.

backend/src/services/FleetSyncService.ts

@@ -303,12 +303,44 @@ export class FleetSyncService {
             db.setSystemState(SYNC_STATE_KEYS.fleetControlIdentity, '');
             db.setSystemState(SYNC_STATE_KEYS.receivedPushedAt('scan_policies'), '');
             db.setSystemState(SYNC_STATE_KEYS.receivedPushedAt('cve_suppressions'), '');
-            db.replaceReplicatedScanPolicies([]);
-            db.replaceReplicatedCveSuppressions([]);
+            db.clearReplicatedRows();
         });
         FleetSyncService.cachedControlIdentity = null;
     }
 
+    /**
+     * Demote this replica back to a standalone control. Atomic transaction:
+     *   - flips `fleet_role` to 'control'
+     *   - clears `fleet_self_identity`, `fleet_control_identity`, and both
+     *     `received_pushed_at:*` watermarks
+     *   - drops every replicated_from_control row from scan_policies and
+     *     cve_suppressions
+     *   - nulls out any orphaned policy_evaluation cache
+     *
+     * After this returns, local writes to scan_policies and cve_suppressions
+     * succeed again (`blockIfReplica` no longer trips).
+     *
+     * Returns `false` if the instance is already a control (no work done);
+     * the route translates that to 409 so the UI doesn't show a misleading
+     * success toast.
+     */
+    public demote(): boolean {
+        const db = DatabaseService.getInstance();
+        if (FleetSyncService.getRole() === 'control') {
+            return false;
+        }
+        db.transaction(() => {
+            db.setSystemState(SYNC_STATE_KEYS.fleetRole, 'control');
+            db.setSystemState(SYNC_STATE_KEYS.fleetSelfIdentity, '');
+            db.setSystemState(SYNC_STATE_KEYS.fleetControlIdentity, '');
+            db.setSystemState(SYNC_STATE_KEYS.receivedPushedAt('scan_policies'), '');
+            db.setSystemState(SYNC_STATE_KEYS.receivedPushedAt('cve_suppressions'), '');
+            db.clearReplicatedRows();
+        });
+        FleetSyncService.cachedControlIdentity = null;
+        return true;
+    }
+
     /**
      * Chain a push behind any in-flight push for the same node. Different
      * nodes still run in parallel via the outer Promise.all.