feat: migrate from SQLite to PostgreSQL - complete schema and API updates

- Remove better-sqlite3, add pg driver
- Convert schema to PostgreSQL types (serial, timestamp, boolean, jsonb)
- Generate fresh PostgreSQL migrations
- Update database connection layer with pg.Pool
- Fix all API routes: remove JSON.parse/stringify, use native timestamps and booleans
- Update drizzle.config.ts and .env.example for PostgreSQL

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

@@ -71,11 +71,10 @@ export async function POST(request: NextRequest) {
         confidence: confidence || null,
         outcome: outcome || null,
         notes: notes || null,
-        sub_spans: sub_spans ? JSON.stringify(sub_spans) : null,
+        sub_spans: 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),
+        model_prediction: model_prediction || null,
       })
       .returning();