fix: align Playwright browser path in Docker

Set PLAYWRIGHT_BROWSERS_PATH=0 in the backend image so Playwright installs Chromium into the same local node_modules path that runtime uses. This prevents missing executable errors during test validation inside containers.

Author: codex

backend/Dockerfile

@@ -11,6 +11,7 @@ COPY backend/ ./backend/
 COPY .env.example .env
 
 ENV DATABASE_URL=postgresql://postgres:postgres@localhost:5432/playwright_testing
+ENV PLAYWRIGHT_BROWSERS_PATH=0
 
 RUN cd backend && npx prisma generate
 RUN cd backend && npx tsc -p tsconfig.json