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>

Dockerfile.backend

@@ -45,6 +45,7 @@ RUN mkdir -p packages/backend
 COPY --from=builder --chown=node:node /app/packages/backend/dist ./packages/backend/dist
 COPY --from=builder --chown=node:node /app/packages/backend/node_modules ./packages/backend/node_modules
 COPY --from=builder --chown=node:node /app/packages/backend/package.json ./packages/backend/package.json
+COPY --from=builder --chown=node:node /app/packages/backend/src/migrations ./packages/backend/migrations
 
 RUN mkdir -p /data/uploads && chown node:node /data/uploads