feat(api): add span annotation export and feedback loop support

- Add GET /api/span-annotations/export endpoint for ML pipeline JSON/CSV export
- Add source and model_prediction fields to span_annotations schema
- Update POST endpoint to accept source (human/model/human_correction) and model_prediction metadata
- Support negative annotations (label 'O' for user corrections to model predictions)
- Create migration 0005 for new schema fields

Completes tasks 8.1-8.4 of candle-backend change

src/app/api/span-annotations/route.ts

@@ -46,6 +46,8 @@ export async function POST(request: NextRequest) {
       notes,
       sub_spans,
       color,
+      source,
+      model_prediction,
     } = body;
 
     if (!chart_id || start_time === undefined || end_time === undefined || !label) {
@@ -71,6 +73,8 @@ export async function POST(request: NextRequest) {
         notes: notes || null,
         sub_spans: sub_spans ? JSON.stringify(sub_spans) : null,
         color: color || '#2196F3',
+        source: source || 'human', // 'human', 'model', or 'human_correction'
+        model_prediction: model_prediction ? JSON.stringify(model_prediction) : null,
         created_at: Math.floor(Date.now() / 1000),
       })
       .returning();