fix: switch backend to CommonJS, generate Drizzle migrations, add migrate script

- Remove "type": "module" and switch tsconfig to CommonJS/Node resolution
  to fix drizzle-kit ESM/CJS incompatibility
- Strip .js extensions from all backend TypeScript imports
- Fix gamification resolver: combine two .where() calls using and()
- Fix index.ts: wrap top-level awaits in async main(), fix Fastify+yoga
  request handling via handleNodeRequestAndResponse
- Generate initial Drizzle SQL migration (0000_pale_hellion.sql) for all
  15 tables
- Add src/scripts/migrate.ts: programmatic Drizzle migrator for production
- Copy migrations folder into Docker image (Dockerfile.backend)
- Add schema:migrate npm script

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/backend/src/db/connection.ts

@@ -1,6 +1,6 @@
 import { drizzle } from "drizzle-orm/node-postgres";
 import { Pool } from "pg";
-import * as schema from "./schema/index.js";
+import * as schema from "./schema/index";
 
 const pool = new Pool({
   connectionString: process.env.DATABASE_URL || "postgresql://sexy:sexy@localhost:5432/sexy",