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/tests/test_rate_limit_proxy.py

@@ -0,0 +1,65 @@
+"""Tests for RateLimitMiddleware honoring proxy headers (X-Forwarded-For)."""
+from middleware.rate_limit import RateLimitMiddleware
+
+
+class _FakeRequest:
+    """Minimal stand-in for starlette.requests.Request."""
+
+    def __init__(self, headers: dict[str, str], host: str | None = "10.0.0.1"):
+        self.headers = headers
+
+        class _Client:
+            pass
+
+        if host is None:
+            self.client = None
+        else:
+            self.client = _Client()
+            self.client.host = host
+
+
+def test_client_ip_uses_x_forwarded_for_first_hop():
+    req = _FakeRequest(
+        headers={"x-forwarded-for": "203.0.113.5, 10.0.0.2"},
+        host="10.0.0.1",
+    )
+    assert RateLimitMiddleware._client_ip(req) == "203.0.113.5"
+
+
+def test_client_ip_strips_whitespace():
+    req = _FakeRequest(headers={"x-forwarded-for": "  198.51.100.7  "})
+    assert RateLimitMiddleware._client_ip(req) == "198.51.100.7"
+
+
+def test_client_ip_falls_back_to_x_real_ip():
+    req = _FakeRequest(headers={"x-real-ip": "203.0.113.99"}, host="10.0.0.1")
+    assert RateLimitMiddleware._client_ip(req) == "203.0.113.99"
+
+
+def test_client_ip_falls_back_to_request_client_host():
+    req = _FakeRequest(headers={}, host="172.18.0.5")
+    assert RateLimitMiddleware._client_ip(req) == "172.18.0.5"
+
+
+def test_client_ip_returns_unknown_without_client():
+    req = _FakeRequest(headers={}, host=None)
+    assert RateLimitMiddleware._client_ip(req) == "unknown"
+
+
+def test_x_forwarded_for_overrides_x_real_ip():
+    req = _FakeRequest(
+        headers={
+            "x-forwarded-for": "203.0.113.5",
+            "x-real-ip": "10.0.0.2",
+        },
+        host="10.0.0.1",
+    )
+    assert RateLimitMiddleware._client_ip(req) == "203.0.113.5"
+
+
+def test_empty_x_forwarded_for_falls_through():
+    req = _FakeRequest(
+        headers={"x-forwarded-for": "", "x-real-ip": "203.0.113.42"},
+        host="10.0.0.1",
+    )
+    assert RateLimitMiddleware._client_ip(req) == "203.0.113.42"