feat: proxy backend API through Next.js using runtime BACKEND_URL

Previously, NEXT_PUBLIC_API_URL was a build-time Next.js variable, making
it impossible to configure the backend URL in docker-compose `environment`
without rebuilding the image.

This change introduces a proper server-side proxy:
- next.config.ts: adds a rewrite rule that forwards all /api/* requests
  to BACKEND_URL (read at server startup, not baked at build time).
  Defaults to http://localhost:8000 so local dev works without config.
- api.ts: API_BASE is now an empty string — all fetch calls use relative
  /api/... paths, which the Next.js server proxies to the backend.
- docker-compose.yml: replaces NEXT_PUBLIC_API_URL build arg with a
  runtime BACKEND_URL env var defaulting to http://backend:8000, using
  Docker's internal networking. Port 8000 no longer needs to be exposed.
- README: updates Docker setup docs, standalone compose example, and
  environment variable reference to reflect BACKEND_URL.

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

docker-compose.yml

@@ -21,13 +21,13 @@ services:
   frontend:
     build:
       context: ./frontend
-      args:
-        # Optional: set this to your backend's external URL if using custom ports
-        # e.g. http://192.168.1.50:9096 — leave empty to auto-detect from browser
-        NEXT_PUBLIC_API_URL: ${NEXT_PUBLIC_API_URL:-}
     container_name: shadowbroker-frontend
     ports:
       - "3000:3000"
+    environment:
+      # Points the Next.js server-side proxy at the backend container via Docker networking.
+      # Change this if your backend runs on a different host or port.
+      - BACKEND_URL=${BACKEND_URL:-http://backend:8000}
     depends_on:
       - backend
     restart: unless-stopped