Reports/Export builder (CSV/XLSX), plus a security fix pass

Adds the Berichte export pipeline (/api/export/{report,events,employees})
with shared CSV/XLSX writers in lib/export.ts and lib/reports-data.ts.

Security pass alongside it: sanitize .or() search terms against PostgREST
filter injection, sanitize spreadsheet cells against CSV/Excel formula
injection, stop leaking raw DB error messages to clients, harden the
service-role client with server-only, add baseline security headers, and
bump the vulnerable nested postcss via an override.

next.config.ts

@@ -1,7 +1,25 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
-  /* config options here */
+  // Baseline security headers (clickjacking, MIME-sniffing, referrer leakage,
+  // browser feature access). No Content-Security-Policy yet: this app has no
+  // inventory of its script/style/connect sources, and shipping a guessed
+  // CSP risks silently breaking Next.js hydration or the Supabase client —
+  // TODO revisit once the actual source list is audited.
+  async headers() {
+    return [
+      {
+        source: "/:path*",
+        headers: [
+          { key: "X-Frame-Options", value: "SAMEORIGIN" },
+          { key: "X-Content-Type-Options", value: "nosniff" },
+          { key: "Referrer-Policy", value: "strict-origin-when-cross-origin" },
+          { key: "Permissions-Policy", value: "camera=(), microphone=(), geolocation=()" },
+          { key: "Strict-Transport-Security", value: "max-age=63072000; includeSubDomains" },
+        ],
+      },
+    ];
+  },
 };
 
 export default nextConfig;