perf: scale workers + per-tablet rate limiting for 20 concurrent users

The default 2-worker gunicorn could only serve 2 concurrent tablet requests,
queueing the rest, and the rate limiter saw every tablet as the same Nginx
container IP, so 20 users would have collectively burned through the
100 req/min general bucket.

- gunicorn: 5 workers x 4 gthread, --forwarded-allow-ips=*, access log
- uvicorn: 4 workers, --proxy-headers, --forwarded-allow-ips=*
- RateLimitMiddleware: resolve real client IP from
  X-Forwarded-For -> X-Real-IP -> request.client.host
- Bump rate_limit_general 100 -> 300 req/min/IP (per tablet now)
- Flask: ProxyFix(x_for=1, x_proto=1, x_host=1) so request.remote_addr
  is the tablet IP, not the Nginx IP
- APIClient: forward X-Forwarded-For + X-Real-IP to FastAPI for both
  JSON and multipart/files calls; safe no-op outside request context
- 12 new tests (7 server + 5 client) covering header precedence,
  forwarding behavior and ProxyFix install

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client/Dockerfile

@@ -22,4 +22,12 @@ RUN pybabel compile -d translations
 
 EXPOSE 5000
 
-CMD ["gunicorn", "--workers", "2", "--bind", "0.0.0.0:5000", "app:create_app()"]
+CMD ["gunicorn", \
+     "--workers", "5", \
+     "--threads", "4", \
+     "--worker-class", "gthread", \
+     "--timeout", "60", \
+     "--bind", "0.0.0.0:5000", \
+     "--access-logfile", "-", \
+     "--forwarded-allow-ips", "*", \
+     "app:create_app()"]