security: full hardening pass — auth guards, rate limiting, headers, internal secret

- Add requireAdmin() to all unprotected admin server actions
  (clients/new, clients/[id], timer-actions — 17 functions total)
- Protect /api/internal/* endpoints with X-Internal-Secret header
  (proxy.ts sends it; routes reject requests without it)
- Randomize auto-generated client slugs with 4-char suffix
  to prevent enumeration via predictable name-based slugs
- Add in-memory rate limiting to /api/client/approve (20/min)
  and /api/client/comment (10/min) per IP
- Add security headers: X-Frame-Options, X-Content-Type-Options,
  Referrer-Policy, Permissions-Policy, X-DNS-Prefetch-Control
- Reduce JWT session from 30 days to 7 days with daily rotation
- Remove hardcoded NEXTAUTH_URL from Dockerfile (pass via Coolify env)
- Genericize client API error messages to not leak data structure
- Update .env.example with all required variables including INTERNAL_SECRET

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

next.config.ts

@@ -1,7 +1,18 @@
 import type { NextConfig } from "next";
 
+const securityHeaders = [
+  { key: "X-Frame-Options", value: "DENY" },
+  { key: "X-Content-Type-Options", value: "nosniff" },
+  { key: "Referrer-Policy", value: "strict-origin-when-cross-origin" },
+  { key: "Permissions-Policy", value: "camera=(), microphone=(), geolocation=()" },
+  { key: "X-DNS-Prefetch-Control", value: "on" },
+];
+
 const nextConfig: NextConfig = {
   output: "standalone",
+  async headers() {
+    return [{ source: "/(.*)", headers: securityHeaders }];
+  },
 };
 
 export default nextConfig;