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/analytics-schema.ts

@@ -51,6 +51,8 @@ export const VISITOR_ANALYTICS_DDL = `
     utm_source TEXT,
     utm_medium TEXT,
     utm_campaign TEXT,
+    stage TEXT,
+    host TEXT,
     FOREIGN KEY (user_id) REFERENCES User(id) ON DELETE SET NULL
   )
 `;
@@ -79,6 +81,18 @@ export const VISITOR_ANALYTICS_ADDED_COLUMNS = [
   {
     name: "utm_campaign",
     ddl: `ALTER TABLE VisitorAnalytics ADD COLUMN utm_campaign TEXT`
+  },
+  {
+    // Which funnel stage a download row is: "click" (a door) or "served"
+    // (the file route). Null on every non-download row (FRE-366).
+    name: "stage",
+    ddl: `ALTER TABLE VisitorAnalytics ADD COLUMN stage TEXT`
+  },
+  {
+    // The Host the client addressed, e.g. "nook.freno.me" — the site
+    // dimension the table never had (FRE-366).
+    name: "host",
+    ddl: `ALTER TABLE VisitorAnalytics ADD COLUMN host TEXT`
   }
 ];
 
@@ -110,7 +124,9 @@ export const VISITOR_ANALYTICS_COLUMNS = [
   "inp",
   "ttfb",
   "dom_load",
-  "load_complete"
+  "load_complete",
+  "stage",
+  "host"
 ] as const;
 
 /**