Add system health dashboard and split settings into separate cards

Add admin-only system health API and UI showing database stats, TMDB
connection status, background job history, storage usage, and environment
variables (with redacted secrets). Split monolithic backup card into
three focused cards (backups, schedule, restore) with dedicated section
headers for Server, Security, and Backups.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/api/admin/system-health/route.ts

@@ -0,0 +1,15 @@
+import { headers } from "next/headers";
+import { NextResponse } from "next/server";
+import { auth } from "@/lib/auth/server";
+import { getSystemHealth } from "@/lib/services/system-health";
+
+export async function GET() {
+  const session = await auth.api.getSession({ headers: await headers() });
+  if (!session)
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  if (session.user.role !== "admin")
+    return NextResponse.json({ error: "Forbidden" }, { status: 403 });
+
+  const health = await getSystemHealth();
+  return NextResponse.json(health);
+}