docs(proxy): say TRUST_CF_CONNECTING_IP needs a Cloudflare-only origin

The flag makes the app believe CF-Connecting-IP from every connection,
before the peer check and whatever TRUST_PROXY_HOPS says, which is safe
only when nothing but Cloudflare can reach the origin. The reverse-proxy
page now says so with the ways to get there, the compose comment and both
example files carry the same condition, and the variable is in the env
manifest. The page's list of peers whose forwarding headers are read now
matches the code: loopback, 0.0.0.0/8, private, CGNAT, link-local,
unique-local and site-local, the reserved blocks and multicast. Those last
two stay trusted, since no internet host can connect from them; a test
pins every listed range and the Cloudflare precedence.

src/lib/__tests__/get-client-ip.test.ts

@@ -407,6 +407,48 @@ describe("getClientIp against the connection's own peer", () => {
     ).toBe("203.0.113.6");
   });
 
+  // The ranges docs/self-hosting/reverse-proxy.md lists, one address each.
+  // Reserved and multicast stay in: no host on the internet can complete a
+  // TCP handshake from them, so a peer there is on a network the operator
+  // runs, like a private one.
+  it("reads forwarding headers from every range the reverse-proxy page lists", () => {
+    for (const proxy of [
+      "127.0.0.2", // loopback
+      "0.0.0.1", // 0.0.0.0/8
+      "10.1.2.3", // private
+      "172.31.0.9",
+      "192.168.1.20",
+      "100.64.0.1", // CGNAT (Tailscale)
+      "169.254.10.10", // link-local
+      "192.0.0.8", // IETF protocol assignments
+      "198.18.0.4", // benchmarking
+      "240.0.0.1", // reserved
+      "fd7a:115c:a1e0::1", // unique-local (Tailscale)
+      "fe80::1", // link-local
+      "fec0::1", // site-local
+      "::ffff:10.0.0.3", // IPv4-mapped private
+    ]) {
+      expect(
+        getClientIp(stamped(proxy, { "x-forwarded-for": "203.0.113.7" })),
+        proxy,
+      ).toBe("203.0.113.7");
+    }
+    for (const outside of ["8.8.8.8", "2001:4860:4860::8888"]) {
+      expect(
+        getClientIp(stamped(outside, { "x-forwarded-for": "203.0.113.7" })),
+      ).toBe(outside);
+    }
+  });
+
+  it("believes cf-connecting-ip from any peer once the flag is on, which is why the origin must take Cloudflare only", () => {
+    process.env.TRUST_CF_CONNECTING_IP = "1";
+    expect(
+      getClientIp(
+        stamped("198.51.100.7", { "cf-connecting-ip": "203.0.113.9" }),
+      ),
+    ).toBe("203.0.113.9");
+  });
+
   it("reads an IPv4 peer that the socket reports in IPv6 form", () => {
     expect(
       getClientIp(