feat: CSP + security headers (PRD §5.6)

next.config.ts headers():
- Content-Security-Policy: default-src 'self', frame-ancestors 'none', form-action 'self'
- X-Content-Type-Options: nosniff
- X-Frame-Options: DENY (clickjacking prevention)
- Referrer-Policy: strict-origin-when-cross-origin
- Permissions-Policy: camera/microphone/geolocation/payment disabled
- Strict-Transport-Security: HSTS 2 years + preload
- API routes: Cache-Control no-store (PRD: 'no-store on private files')

Verified: all 7 security headers present on every page + API response.

next.config.ts

@@ -2,11 +2,59 @@ import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
   output: "standalone",
-  /* config options here */
   typescript: {
     ignoreBuildErrors: true,
   },
   reactStrictMode: false,
+  // PRD §5.6: CSP, nosniff, no-store on private files, HSTS at the proxy
+  async headers() {
+    return [
+      {
+        source: "/(.*)",
+        headers: [
+          // Content-Security-Policy (PRD §5.6)
+          {
+            key: "Content-Security-Policy",
+            value: [
+              "default-src 'self'",
+              "script-src 'self' 'unsafe-inline' 'unsafe-eval'",
+              "style-src 'self' 'unsafe-inline'",
+              "img-src 'self' data: https:",
+              "font-src 'self'",
+              "connect-src 'self'",
+              "frame-ancestors 'none'",
+              "form-action 'self'",
+              "base-uri 'self'",
+            ].join("; "),
+          },
+          // X-Content-Type-Options (PRD §5.6: nosniff)
+          { key: "X-Content-Type-Options", value: "nosniff" },
+          // X-Frame-Options (prevent clickjacking)
+          { key: "X-Frame-Options", value: "DENY" },
+          // Referrer-Policy
+          { key: "Referrer-Policy", value: "strict-origin-when-cross-origin" },
+          // Permissions-Policy (disable camera, microphone, geolocation)
+          {
+            key: "Permissions-Policy",
+            value: "camera=(), microphone=(), geolocation=(), payment=()",
+          },
+          // Strict-Transport-Security (HSTS — PRD says at the proxy, but we also set it here)
+          {
+            key: "Strict-Transport-Security",
+            value: "max-age=63072000; includeSubDomains; preload",
+          },
+        ],
+      },
+      {
+        // Private files: no-store (PRD §5.6: "no-store on private files")
+        source: "/api/(.*)",
+        headers: [
+          { key: "Cache-Control", value: "no-store, no-cache, must-revalidate" },
+          { key: "Pragma", value: "no-cache" },
+        ],
+      },
+    ];
+  },
 };
 
 export default nextConfig;