feat: locked-feature upgrade prompts route to billing + track upgrade interest (v0.81.0)

UpgradeDialog's CTA now sends users to Settings > Billing (with a feature-specific banner) instead of a generic support-ticket prefill, and fires a best-effort tracking beacon recording which feature triggered the click. New feature_upgrade_clicks table + Admin > Insights chart ("Most-requested locked features") gives admins visibility into which locked features actually drive upgrade interest.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/app/api/v1/users/me/upgrade-interest/route.ts

@@ -0,0 +1,34 @@
+import { NextRequest, NextResponse } from "next/server";
+import { db, featureUpgradeClicks } from "@epicure/db";
+import { requireSession } from "@/lib/api-auth";
+import { applyRateLimit } from "@/lib/rate-limit";
+import { z } from "zod";
+
+const Schema = z.object({
+  featureKey: z.string().min(1).max(100),
+});
+
+// Fired (best-effort, fire-and-forget from the client) when a user clicks
+// "I'm interested" on a locked-feature upgrade dialog — purely a signal for
+// Admin -> Insights ("most-requested locked features"), not a purchase or
+// a support ticket. Never blocks the redirect to billing on the client.
+export async function POST(req: NextRequest) {
+  const { session, response } = await requireSession();
+  if (response) return response;
+
+  const limited = await applyRateLimit(`rl:upgrade-interest:${session!.user.id}`, 20, 60);
+  if (limited) return limited;
+
+  const parsed = Schema.safeParse(await req.json().catch(() => null));
+  if (!parsed.success) {
+    return NextResponse.json({ error: "Invalid request" }, { status: 400 });
+  }
+
+  await db.insert(featureUpgradeClicks).values({
+    id: crypto.randomUUID(),
+    userId: session!.user.id,
+    featureKey: parsed.data.featureKey,
+  });
+
+  return NextResponse.json({ ok: true }, { status: 201 });
+}