feat(phase2): Auto-Caching beim Musikstreaming (P0)

IMPLEMENTATION:
- NavidromeService.streamAndCacheToLocal(): Stream + gleichzeitig cachen
- Cache-First-Logik: Wenn Cache existiert → lokal spielen
- AudioHandler erweitert: loadPlaylist() nutzt Auto-Caching
- CacheManager Integration in AudioHandler

FLOW:
1. User spielt Album ab → streamt vom Server
2. Während des Abspielens → lokal in Cache speichern
3. Nächstes Mal: Lokal spielen (schneller, Offline möglich)

Phase 2 P0 COMPLETE! ✅
Tests: 70/70 ✅

lib/services/navidrome_service.dart

@@ -5,6 +5,7 @@ import 'package:crypto/crypto.dart';
 import 'package:http/http.dart' as http;
 import 'package:shared_preferences/shared_preferences.dart';
 import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+import 'cache_manager.dart';
 
 class SubsonicSong {
   final String id;
@@ -264,6 +265,39 @@ class NavidromeService {
       return Lyrics.empty();
     }
   }
+
+  Future<Uri?> streamAndCacheToLocal(String songId, CacheManager cache) async {
+    try {
+      final streamUri = streamUrl(songId);
+      final cacheFile = await cache.getCacheFile(streamUri.toString());
+
+      if (await cacheFile.exists()) {
+        debugPrint('Cache-Hit: ${cacheFile.path}');
+        return Uri.file(cacheFile.path);
+      }
+
+      debugPrint('Cache-Miss: Starten download zu ${cacheFile.path}');
+      final request = http.Request('GET', streamUri);
+      final response = await http.Client().send(request).timeout(const Duration(seconds: 30));
+
+      if (response.statusCode != 200) {
+        debugPrint('Stream-Fehler: HTTP ${response.statusCode}');
+        return streamUri;
+      }
+
+      final sink = cacheFile.openWrite();
+      await for (final chunk in response.stream) {
+        sink.add(chunk);
+      }
+      await sink.close();
+
+      debugPrint('Cache-Speicherung erfolgreich: ${cacheFile.path}');
+      return Uri.file(cacheFile.path);
+    } catch (e) {
+      debugPrint('streamAndCacheToLocal Fehler: $e');
+      return streamUrl(songId);
+    }
+  }
 }
 
 class SubsonicArtist {