fix(docker): align Dockerfile port with .env configuration (8521)

- Updated EXPOSE from 8080 to 8521
- Updated HEALTHCHECK to check port 8521
- Updated default ENV PORT from 8080 to 8521

This fixes the deployment health check issue where the container
was expecting port 8080 but the app was configured to run on 8521.

backend/Dockerfile

@@ -54,14 +54,14 @@ RUN chown -R appuser:appgroup /app
 USER appuser
 
 # Expose port
-EXPOSE 8080
+EXPOSE 8521
 
 # Health check
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
-    CMD wget -qO- http://localhost:8080/health || exit 1
+    CMD wget -qO- http://localhost:8521/health || exit 1
 
 # Environment defaults
-ENV PORT=8080 \
+ENV PORT=8521 \
     TZ=America/Sao_Paulo
 
 CMD ["./main"]