fix(api): correctly serialize nested objects for db insertion

When creating new headlines or sources, the API was failing with a
database error because it tried to insert nested `source`, `category`,
and `headquarters` objects directly into the database. The database
schema expects flattened foreign key columns (`source_id`, `category_id`,
`headquarters_country_id`).

This fix updates the `toJson` functions provided during the setup of
the `headlineRepository` and `sourceRepository` in `AppDependencies`.
These custom functions now correctly flatten the nested objects into
their respective ID fields before the data is sent to the database
client, resolving the "column does not exist" error.

Author: fulleni

lib/src/config/app_dependencies.dart

@@ -115,7 +115,21 @@ class AppDependencies {
         }
         return Headline.fromJson(json);
       },
-      (h) => h.toJson(), // toJson already handles DateTime correctly
+      (headline) {
+        final json = headline.toJson();
+        // The database expects source_id and category_id, not nested objects.
+        // We extract the IDs and remove the original objects to match the
+        // schema.
+        if (headline.source != null) {
+          json['source_id'] = headline.source!.id;
+        }
+        if (headline.category != null) {
+          json['category_id'] = headline.category!.id;
+        }
+        json.remove('source');
+        json.remove('category');
+        return json;
+      },
     );
     categoryRepository = _createRepository(
       connection,
@@ -147,7 +161,15 @@ class AppDependencies {
         }
         return Source.fromJson(json);
       },
-      (s) => s.toJson(),
+      (source) {
+        final json = source.toJson();
+        // The database expects headquarters_country_id, not a nested object.
+        // We extract the ID and remove the original object to match the
+        // schema.
+        json['headquarters_country_id'] = source.headquarters?.id;
+        json.remove('headquarters');
+        return json;
+      },
     );
     countryRepository = _createRepository(
       connection,