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/tests/test_api_client_proxy.py

@@ -0,0 +1,70 @@
+"""Tests that APIClient forwards the real tablet IP via X-Forwarded-For."""
+from unittest.mock import patch
+
+from services.api_client import api_client
+
+
+def _last_call_headers(mock):
+    """Return the headers kwarg from the most recent requests.* call."""
+    return mock.call_args.kwargs["headers"]
+
+
+def test_get_forwards_real_client_ip(flask_app):
+    with flask_app.test_request_context(
+        "/", environ_base={"REMOTE_ADDR": "203.0.113.10"}
+    ):
+        with patch("services.api_client.requests.get") as mock_get:
+            mock_get.return_value.ok = True
+            mock_get.return_value.status_code = 200
+            mock_get.return_value.json.return_value = {}
+            api_client.get("/api/health")
+
+        headers = _last_call_headers(mock_get)
+        assert headers["X-Forwarded-For"] == "203.0.113.10"
+        assert headers["X-Real-IP"] == "203.0.113.10"
+
+
+def test_post_json_forwards_real_client_ip(flask_app):
+    with flask_app.test_request_context(
+        "/", environ_base={"REMOTE_ADDR": "198.51.100.42"}
+    ):
+        with patch("services.api_client.requests.post") as mock_post:
+            mock_post.return_value.ok = True
+            mock_post.return_value.status_code = 200
+            mock_post.return_value.json.return_value = {}
+            api_client.post("/api/foo", data={"x": 1})
+
+        headers = _last_call_headers(mock_post)
+        assert headers["X-Forwarded-For"] == "198.51.100.42"
+
+
+def test_post_with_files_forwards_real_client_ip(flask_app):
+    with flask_app.test_request_context(
+        "/", environ_base={"REMOTE_ADDR": "198.51.100.77"}
+    ):
+        with patch("services.api_client.requests.post") as mock_post:
+            mock_post.return_value.ok = True
+            mock_post.return_value.status_code = 200
+            mock_post.return_value.json.return_value = {}
+            api_client.post(
+                "/api/files/upload", data={}, files={"file": ("x.txt", b"data")}
+            )
+
+        headers = _last_call_headers(mock_post)
+        assert headers["X-Forwarded-For"] == "198.51.100.77"
+        assert headers["X-Real-IP"] == "198.51.100.77"
+
+
+def test_real_client_ip_returns_none_outside_request_context(flask_app):
+    """Background workers without a Flask request must NOT inject a fake IP."""
+    from services.api_client import APIClient
+
+    with flask_app.app_context():
+        assert APIClient._real_client_ip() is None
+
+
+def test_proxy_fix_is_installed(flask_app):
+    """ProxyFix must wrap wsgi_app so REMOTE_ADDR is rewritten in production."""
+    from werkzeug.middleware.proxy_fix import ProxyFix
+
+    assert isinstance(flask_app.wsgi_app, ProxyFix)