feat: SQLCipher-Verschluesselung der pyramid.sqlite (M1) via neue Storage-Fassade app_database.dart

- AppDatabase.open()/openForPush() kapseln Factory, Schluessel (Keystore),
  Klartext->SQLCipher-Migration (Backup/Verify/atomarer Tausch) und PRAGMAs
- Android/Windows/Linux verschluesselt, iOS/macOS bewusst Klartext
- 5 neue Tests mit echter SQLCipher-DLL; Windows-Praxistest mit echter DB
  erfolgreich (23 Tabellen verifiziert, E2EE intakt); Android UNGETESTET

lib/core/background_push.dart

@@ -7,11 +7,9 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter_local_notifications/flutter_local_notifications.dart';
 import 'package:flutter_vodozemac/flutter_vodozemac.dart' as vod;
 import 'package:matrix/matrix.dart';
-import 'package:path/path.dart' as p;
-import 'package:path_provider/path_provider.dart';
+import 'package:pyramid/core/app_database.dart';
 import 'package:pyramid/core/soft_logout_guard.dart';
 import 'package:shared_preferences/shared_preferences.dart';
-import 'package:sqflite/sqflite.dart' as sqflite_native;
 
 /// Port name used to locate the main isolate from background isolates.
 /// Registered by notification_service.dart when the app starts.
@@ -182,18 +180,13 @@ Future<Client?> _buildClient() async {
       await vod.init();
     } catch (_) {}
 
-    final appDir = await getApplicationSupportDirectory();
-    final dbPath = p.join(appDir.path, 'pyramid.sqlite');
-
-    final db = await sqflite_native.databaseFactory
-        .openDatabase(dbPath)
-        .timeout(const Duration(seconds: 5));
-
-    // Prevent "database is locked" if the main process still has the DB open.
-    try {
-      await db.execute('PRAGMA busy_timeout = 5000');
-      await db.execute('PRAGMA journal_mode = WAL');
-    } catch (_) {}
+    // Gemeinsame Storage-Fassade: erkennt Klartext vs. SQLCipher am
+    // Datei-Header, migriert aber NIE selbst (docs/SQLCIPHER_MIGRATION.md).
+    // null = gerade kein sicherer Zugriff (Migration läuft / DB oder
+    // Schlüssel fehlt) → Notification behält den nativen Platzhalter.
+    final db =
+        await AppDatabase.openForPush().timeout(const Duration(seconds: 10));
+    if (db == null) return null;
 
     final sdkDb = await MatrixSdkDatabase.init('pyramid', database: db);