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>

server/config.py

@@ -23,9 +23,9 @@ class Settings(BaseSettings):
     upload_dir: str = "uploads"
     max_upload_size_mb: int = 50
 
-    # Rate Limiting (requests per minute)
+    # Rate Limiting (requests per minute, per real client IP)
     rate_limit_login: int = 5
-    rate_limit_general: int = 100
+    rate_limit_general: int = 300
 
     # SSL (Production)
     ssl_certfile: str | None = None