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/lib/db/schema.ts

@@ -61,5 +61,7 @@ export const spanAnnotations = sqliteTable('span_annotations', {
   notes: text('notes'), // free-text, nullable
   sub_spans: text('sub_spans'), // JSON array of sub-span objects, nullable
   color: text('color').notNull().default('#2196F3'), // hex color
+  source: text('source').notNull().default('human'), // 'human'|'model'|'human_correction'
+  model_prediction: text('model_prediction'), // JSON metadata when confirming/correcting predictions
   created_at: integer('created_at').notNull(),
 });