Implement Linux data models (C/Vala)

- Add FeedItem, Feed, FeedSubscription models
- Add SearchResult, SearchFilters, SearchQuery models
- Add NotificationPreferences, ReadingPreferences models
- Implement JSON serialization/deserialization for all models
- Add equality comparison methods
- Following GNOME HIG naming conventions
- Build system configured with Meson/Ninja

native-route/linux/src/models/namespaces.vala

@@ -0,0 +1,5 @@
+/*
+ * Namespace definition for RSSuper Linux models
+ */
+public namespace RSSuper {
+}