Initial Flutter booru app

lib/core/cache/image_cache_service.dart

@@ -0,0 +1,7 @@
+import '../../backend/models/post.dart';
+
+class ImageCacheService {
+  String previewCacheKey(Post post) => 'preview:${post.providerId}:${post.id}';
+  String sampleCacheKey(Post post) => 'sample:${post.providerId}:${post.id}';
+  String fileCacheKey(Post post) => 'file:${post.providerId}:${post.id}';
+}