fix: resolve proxy gzip decoding and BACKEND_URL Docker override issues

Two bugs introduced by the Next.js proxy Route Handler:

1. ERR_CONTENT_DECODING_FAILED — Node.js fetch() automatically
   decompresses gzip/br responses from the backend, but the proxy was
   still forwarding Content-Encoding and Content-Length headers to the
   browser. The browser would then try to decompress already-decompressed
   data and fail. Fixed by stripping Content-Encoding and Content-Length
   from upstream response headers.

2. BACKEND_URL shell env leak into Docker Compose — docker-compose.yml
   used ${BACKEND_URL:-http://backend:8000}, which was being overridden
   by BACKEND_URL=http://localhost:8000 set in .mise.local.toml for local
   dev. Inside the frontend container, localhost:8000 does not exist,
   causing all proxied requests to return 502. Fixed by hardcoding
   http://backend:8000 in docker-compose.yml so the shell environment
   cannot override it.

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

docker-compose.yml

@@ -27,7 +27,7 @@ services:
     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}
+      - BACKEND_URL=http://backend:8000
     depends_on:
       - backend
     restart: unless-stopped