🔧 Fix: Add DATABASE_URL runtime support for category pages

- Updated Dockerfile to accept DATABASE_URL at runtime
- Added .env.example for frontend
- Created Coolify environment setup guide

Fixes category pages 404 error - DATABASE_URL needs to be set in Coolify

Dockerfile

@@ -27,6 +27,10 @@ WORKDIR /app
 ENV NODE_ENV=production
 ENV NEXT_TELEMETRY_DISABLED=1
 
+# Database URL will be provided at runtime by Coolify
+ARG DATABASE_URL
+ENV DATABASE_URL=${DATABASE_URL}
+
 RUN addgroup --system --gid 1001 nodejs
 RUN adduser --system --uid 1001 nextjs