fix(api): correct syntax and constraints in headlines schema

The server was failing to start due to a syntax error (a missing comma) in the `CREATE TABLE` statement for the `headlines` table.

Additionally, the schema incorrectly defined the `image_url`, `url`, and `published_at` columns as `NOT NULL`, which contradicted the nullable fields in the `Headline` data model.

This change corrects the syntax error and removes the incorrect `NOT NULL` constraints, fully aligning the database schema with the model and resolving the startup failure.

Author: fulleni

lib/src/services/database_seeding_service.dart

@@ -107,13 +107,13 @@ class DatabaseSeedingService {
             title TEXT NOT NULL,
             source_id TEXT NOT NULL,
             category_id TEXT NOT NULL,
-            image_url TEXT NOT NULL,
-            url TEXT NOT NULL,
-            published_at TIMESTAMPTZ NOT NULL,
+            image_url TEXT,
+            url TEXT,
+            published_at TIMESTAMPTZ,
             description TEXT,
             content TEXT,
             created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
-            updated_at TIMESTAMPTZ
+            updated_at TIMESTAMPTZ,
             status TEXT,
             type TEXT
           );