feat: Admin Insights page — charts for signups, recipes, tiers, usage (v0.55.0)

New Admin > Insights: signups/day and recipes-created/day (manual vs AI)
over the last 30 days, users by tier, recipes by visibility, monthly AI
call totals (6 months), and support tickets by status.

Two hand-rolled SVG chart components (bar-chart.tsx, time-series-chart.tsx)
instead of pulling in a charting library — avoids a new dependency and
any React 19 peer-dep risk. Both ship a hover tooltip (crosshair for the
time series, per-bar for the bar chart) and a "show as table" toggle per
the dataviz method's accessibility requirement.

Repurposed the app's existing (previously unused, grayscale-only)
shadcn --chart-1..5 CSS variables with a validated categorical palette —
run through the dataviz skill's CVD/contrast validator for both light and
dark surfaces (both pass; three light-mode slots fall under 3:1 contrast
by design, mitigated by the charts' direct value/axis labels).

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

apps/web/app/admin/layout.tsx

@@ -3,11 +3,12 @@ import { headers } from "next/headers";
 import { auth } from "@/lib/auth/server";
 import { db, users, eq } from "@epicure/db";
 import Link from "next/link";
-import { Shield, Users, BookOpen, Settings, BarChart3, ClipboardList, HardDrive, Bot, ArrowLeft, Gauge, Mail, Flag, History, LifeBuoy } from "lucide-react";
+import { Shield, Users, BookOpen, Settings, BarChart3, ClipboardList, HardDrive, Bot, ArrowLeft, Gauge, Mail, Flag, History, LifeBuoy, TrendingUp } from "lucide-react";
 import { cn } from "@/lib/utils";
 
 const adminNav = [
   { href: "/admin", label: "Overview", icon: BarChart3 },
+  { href: "/admin/insights", label: "Insights", icon: TrendingUp },
   { href: "/admin/users", label: "Users", icon: Users },
   { href: "/admin/invites", label: "Invites", icon: Mail },
   { href: "/admin/recipes", label: "Recipes", icon: BookOpen },