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

drizzle.config.ts

@@ -3,8 +3,8 @@ import { defineConfig } from 'drizzle-kit';
 export default defineConfig({
   schema: './src/lib/db/schema.ts',
   out: './drizzle',
-  dialect: 'sqlite',
+  dialect: 'postgresql',
   dbCredentials: {
-    url: './data/candles.db',
+    url: process.env.DATABASE_URL!,
   },
 });