feat: add database schema, migrations, and API endpoints for span annotations

- Add span_label_types and span_annotations tables to schema
- Seed default span label types (bull_flag, bear_flag, etc.)
- Implement CRUD API endpoints for span label types
- Implement CRUD API endpoints for span annotations
- Add time swap validation in POST endpoint (start_time <= end_time)

scripts/seed-span-labels.ts

@@ -0,0 +1,11 @@
+import { seedSpanLabelTypes } from '../src/lib/db/seed-span-label-types';
+
+seedSpanLabelTypes()
+  .then(() => {
+    console.log('Seed complete');
+    process.exit(0);
+  })
+  .catch((error) => {
+    console.error('Seed failed:', error);
+    process.exit(1);
+  });