nostrdb: close database when backgrounded

Otherwise iOS gets mad because we are holding onto a lockfile in a
shared container which is apparently not allowed.

Fixes: a1e6be214e ("Migrate NostrDB files to shared app group file container")

damus/ContentView.swift

@@ -448,18 +448,25 @@ struct ContentView: View {
                 break
             }
         }
+        .onReceive(NotificationCenter.default.publisher(for: UIApplication.willEnterForegroundNotification)) { obj in
+            print("📙 DAMUS ACTIVE NOTIFY")
+            try? damus_state.ndb.reopen()
+        }
         .onChange(of: scenePhase) { (phase: ScenePhase) in
+            guard let damus_state else { return }
             switch phase {
             case .background:
                 print("📙 DAMUS BACKGROUNDED")
+                Task { @MainActor in
+                    damus_state.ndb.close()
+                }
                 break
             case .inactive:
                 print("📙 DAMUS INACTIVE")
                 break
             case .active:
                 print("📙 DAMUS ACTIVE")
-                guard let ds = damus_state else { return }
-                ds.pool.ping()
+                damus_state.pool.ping()
             @unknown default:
                 break
             }