fix(api): rate-limit per caller, not per proxy

The limiter keyed on request.client.host, which in staging and prod is
the Next container — the backend has no published ports and nothing else
can reach it. So every browser user on the site shared one 60/minute
bucket per route. Measured against staging: 70 concurrent requests to
/api/schools returned exactly 60 OK and 10 refused, from one machine.

CF-Connecting-IP first. Cloudflare fronts both environments and
overwrites any client-supplied value, which a parsed X-Forwarded-For
chain does not guarantee. The XFF fallback is forgeable only from inside
the Docker network.

Named rather than hidden: the shared bucket was an accidental global
throttle on a single-process backend, and correct per-user keying
removes it. A real global ceiling belongs at Cloudflare, which is
already in the path; slowapi cannot express one without a second Limiter
and middleware this app does not install.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

backend/app.py

@@ -296,8 +296,36 @@ def clean_filter_values(series: pd.Series) -> list[str]:
 # SECURITY MIDDLEWARE & HELPERS
 # =============================================================================
 
-# Rate limiter
-limiter = Limiter(key_func=get_remote_address)
+def client_key(request: Request) -> str:
+    """The rate-limit bucket: the real caller, not the proxy in front of them.
+
+    `get_remote_address` reads request.client.host. In staging and production
+    the backend has no published ports and sits on the internal network, so its
+    only caller is the Next proxy — meaning every browser user on the site
+    shared one bucket. Measured before this fix: 70 concurrent requests to
+    /api/schools returned 60 OK and 10 refused.
+
+    CF-Connecting-IP first, because Cloudflare (in front of both environments)
+    sets it on every origin request and *overwrites* any client-supplied value,
+    which a parsed X-Forwarded-For chain does not guarantee. The XFF fallback is
+    forgeable, but only by a caller already inside the Docker network, which is
+    the one place nothing untrusted can reach.
+    """
+    cf = request.headers.get("cf-connecting-ip")
+    if cf:
+        return cf.strip()
+    xff = request.headers.get("x-forwarded-for")
+    if xff:
+        return xff.split(",")[0].strip()
+    return get_remote_address(request)
+
+
+# Rate limiter. No in-app global ceiling: slowapi's default_limits and
+# application_limits are both keyed by key_func (so per-client, not global)
+# and the latter only applies with SlowAPIMiddleware installed, which this app
+# does not use. A global cap belongs at Cloudflare, which is already in the
+# path. See the spec's §1 for why that is deliberate.
+limiter = Limiter(key_func=client_key)
 
 
 class SecurityHeadersMiddleware(BaseHTTPMiddleware):

backend/tests/test_rate_limit_key.py

@@ -0,0 +1,58 @@
+"""The rate-limit bucket must be the caller, not the proxy in front of them.
+
+`get_remote_address` reads request.client.host. In staging and production the
+backend has no published ports and its only caller is the Next proxy, so that
+host is the Next container — one bucket for every browser user on the site.
+Measured before this fix: 70 concurrent requests, 60 served and 10 refused.
+"""
+
+from starlette.datastructures import Headers
+
+from backend.app import client_key
+
+
+class _Req:
+    """Enough of a Request for the key function: headers and a client host."""
+
+    def __init__(self, headers: dict, host: str = "10.0.0.9"):
+        self.headers = Headers(headers)
+        self.client = type("C", (), {"host": host})()
+        self.scope = {"type": "http", "client": (host, 0),
+                      "headers": [(k.lower().encode(), v.encode())
+                                  for k, v in headers.items()]}
+
+
+def test_cloudflare_header_wins():
+    # Cloudflare sets CF-Connecting-IP and overwrites any client-supplied
+    # value, so it is trustworthy in a way a parsed XFF chain is not.
+    assert client_key(_Req({"cf-connecting-ip": "203.0.113.7"})) == "203.0.113.7"
+
+
+def test_forwarded_for_is_the_fallback_and_takes_the_first_entry():
+    # Left-most is the original client; everything after it is proxies.
+    assert client_key(
+        _Req({"x-forwarded-for": "203.0.113.7, 10.0.0.2"})) == "203.0.113.7"
+
+
+def test_remote_address_is_the_last_resort():
+    assert client_key(_Req({}, host="10.0.0.9")) == "10.0.0.9"
+
+
+def test_cloudflare_header_beats_forwarded_for():
+    key = client_key(_Req({"cf-connecting-ip": "203.0.113.7",
+                           "x-forwarded-for": "198.51.100.1"}))
+    assert key == "203.0.113.7"
+
+
+def test_two_callers_get_two_buckets():
+    # The whole point: one user exhausting their limit must not refuse another.
+    a = client_key(_Req({"cf-connecting-ip": "203.0.113.7"}))
+    b = client_key(_Req({"cf-connecting-ip": "203.0.113.8"}))
+    assert a != b
+
+
+def test_whitespace_is_stripped():
+    # "a, b" split on comma leaves a leading space on every entry but the
+    # first; an unstripped key silently creates a second bucket per client.
+    assert client_key(_Req({"x-forwarded-for": " 203.0.113.7 ,10.0.0.2"})) \
+        == "203.0.113.7"