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>

src/lib/auth.ts

@@ -33,7 +33,8 @@ export const authOptions: NextAuthOptions = {
   ],
   session: {
     strategy: "jwt",       // stateless JWT — no DB session table (per D-03)
-    maxAge: 30 * 24 * 60 * 60, // 30 days
+    maxAge: 7 * 24 * 60 * 60,  // 7 days (reduced from 30 for security)
+    updateAge: 24 * 60 * 60,   // rotate token daily on active use
   },
   pages: {
     signIn: "/admin/login",   // custom login page (per D-07)