analytics: make each weekly funnel stage separately measurable (FRE-366)

Gap 1 — click and served share one label: trackDownload now takes a
stage ("click" | "served") carried on a new VisitorAnalytics.stage
column; the Nook door and the tRPC proxy record clicks, the DMG file
route records served. summariseDownloads() and the /analytics downloads
block are untouched — the stage is a separate column, so the path
grouping cannot change.

Gap 2 — checkout start records nothing: the checkout route records a
created Stripe session through the shared trackRequest writer (same
referrer, campaign, country dimensions as every door) under its own
path, and STAGES.checkout reads it instead of the /checkout page proxy.

Gap 3 — loopback addresses, no host dimension: every writer now
resolves the client address through resolveClientIp, which trusts
x-forwarded-for only when the direct peer is the loopback production
proxy (trust boundary stated in request-client.ts), and records the
host on each row through the same site-context extractor routing uses.
stage and host reach the table through VISITOR_ANALYTICS_ADDED_COLUMNS;
the weekly job applies the same bootstrap before reading so it renders
seven stages against a database no deploy has migrated yet.

The weekly read splits downloadClick/dmgServed, scopes visits to
nook.freno.me, and computes its caveats from the window: pre-instrumentation
rows are stated and excluded, and the loopback warning now fires only
when rows recorded since the instrumentation still carry it.

src/server/request-client.test.ts

@@ -0,0 +1,78 @@
+import { beforeEach, describe, expect, it, mock } from "bun:test";
+
+// The trust boundary of FRE-366: x-forwarded-for is client-controlled text,
+// believed only when the direct peer is the loopback production proxy. These
+// tests pin that boundary — a regression here either re-records the proxy as
+// the visitor (unique visitors collapses to one host) or lets any client
+// spoof its address.
+
+let directIp: string | null = null;
+mock.module("vinxi/http", () => ({
+  getRequestIP: () => directIp
+}));
+
+const { resolveClientIp, isLoopbackAddress, requestHost } = await import(
+  "./request-client"
+);
+
+function event(
+  headers: Record<string, string> = {},
+  url = "https://nook.freno.me/downloads"
+) {
+  return {
+    request: new Request(url, { headers }),
+    nativeEvent: {}
+  } as never;
+}
+
+beforeEach(() => {
+  directIp = null;
+});
+
+describe("resolveClientIp", () => {
+  it("takes the first forwarded hop when the direct peer is the loopback proxy", () => {
+    directIp = "127.0.0.1";
+    expect(
+      resolveClientIp(event({ "x-forwarded-for": "198.51.100.9, 10.0.0.1" }))
+    ).toBe("198.51.100.9");
+
+    directIp = "::1";
+    expect(
+      resolveClientIp(event({ "x-forwarded-for": "198.51.100.10" }))
+    ).toBe("198.51.100.10");
+  });
+
+  it("ignores a forwarded header from a real peer — it is spoofable", () => {
+    directIp = "203.0.113.7";
+    expect(
+      resolveClientIp(event({ "x-forwarded-for": "198.51.100.9" }))
+    ).toBe("203.0.113.7");
+  });
+
+  it("keeps the loopback peer when the proxy set no forwarded header", () => {
+    directIp = "::ffff:127.0.0.1";
+    expect(resolveClientIp(event())).toBe("::ffff:127.0.0.1");
+  });
+
+  it("records nothing rather than inventing an address", () => {
+    expect(resolveClientIp(event())).toBeNull();
+  });
+});
+
+describe("isLoopbackAddress", () => {
+  it("covers the v4 range, the shorthand and the v6 forms", () => {
+    expect(isLoopbackAddress("127.0.0.1")).toBe(true);
+    expect(isLoopbackAddress("127.0.0.53")).toBe(true);
+    expect(isLoopbackAddress("::1")).toBe(true);
+    expect(isLoopbackAddress("::ffff:127.0.0.1")).toBe(true);
+    expect(isLoopbackAddress("203.0.113.7")).toBe(false);
+    expect(isLoopbackAddress("10.0.0.1")).toBe(false);
+  });
+});
+
+describe("requestHost", () => {
+  it("reads the host the client addressed from the request URL", () => {
+    expect(requestHost(event())).toBe("nook.freno.me");
+    expect(requestHost(event({}, "https://freno.me/"))).toBe("freno.me");
+  });
+});