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

.env.example

@@ -1,6 +1,6 @@
 NODE_ENV=production
 PORT=3000
-DATABASE_PATH=/app/data/candles.db
+DATABASE_URL=postgresql://ml_user:ml_password@postgres:5432/candle_annotator
 
 # ML Inference Service Configuration
 INFERENCE_API_URL=http://localhost:8001