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.ts

@@ -64,6 +64,10 @@ export interface AnalyticsEntry {
   ttfb?: number | null;
   domLoad?: number | null;
   loadComplete?: number | null;
+  /** Funnel stage for download rows ("click" | "served"); null elsewhere. */
+  stage?: string | null;
+  /** Host the client addressed, e.g. "nook.freno.me" — the site dimension. */
+  host?: string | null;
 }
 
 /**
@@ -124,7 +128,9 @@ async function flushAnalyticsBuffer(): Promise<void> {
           entry.inp || null,
           entry.ttfb || null,
           entry.domLoad || null,
-          entry.loadComplete || null
+          entry.loadComplete || null,
+          entry.stage || null,
+          entry.host || null
         ]
       });
     }
@@ -258,6 +264,8 @@ export async function queryAnalytics(
     utm_medium: row.utm_medium as string | null,
     utm_campaign: row.utm_campaign as string | null,
     duration_ms: row.duration_ms as number | null,
+    stage: row.stage as string | null,
+    host: row.host as string | null,
     created_at: row.created_at as string
   }));
 }