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/matrix_client.dart

@@ -1,5 +1,4 @@
 import 'dart:async';
-import 'dart:io';
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
@@ -7,40 +6,15 @@ import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'package:flutter_vodozemac/flutter_vodozemac.dart' as vod;
 import 'package:matrix/encryption/utils/key_verification.dart';
 import 'package:matrix/matrix.dart';
-import 'package:path/path.dart' as p;
-import 'package:path_provider/path_provider.dart';
-import 'package:sqflite/sqflite.dart' as sqflite_native;
-import 'package:sqflite_common_ffi/sqflite_ffi.dart';
+import 'package:pyramid/core/app_database.dart';
 import 'package:pyramid/core/auth_log.dart';
 import 'package:pyramid/core/soft_logout_guard.dart';
 import 'package:pyramid/core/voip_manager.dart';
 
 final matrixClientProvider = FutureProvider<Client>((ref) async {
-  final DatabaseFactory factory;
-
-  if (!kIsWeb && (Platform.isAndroid || Platform.isIOS)) {
-    // Use the native sqflite plugin — Android/iOS have system SQLite built-in,
-    // no .so loading required.
-    factory = sqflite_native.databaseFactory;
-  } else if (!kIsWeb) {
-    sqfliteFfiInit();
-    factory = databaseFactoryFfi;
-  } else {
-    throw UnsupportedError('Web not supported');
-  }
-
-  final appDir = await getApplicationSupportDirectory();
-  final dbPath = p.join(appDir.path, 'pyramid.sqlite');
-
-  final db = await factory.openDatabase(dbPath);
-  
-  // Optimization for Windows/FFI to prevent "database is locked" errors
-  try {
-    await db.execute('PRAGMA journal_mode=WAL');
-    await db.execute('PRAGMA busy_timeout = 5000');
-  } catch (e) {
-    if (kDebugMode) debugPrint('Could not set PRAGMAs: $e');
-  }
+  // Factory-Wahl, Verschlüsselung (SQLCipher), Migration und PRAGMAs stecken
+  // komplett in der Storage-Fassade — siehe docs/SQLCIPHER_MIGRATION.md.
+  final db = await AppDatabase.open();
 
   final sdkDb = await MatrixSdkDatabase.init('pyramid', database: db);