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/proxy.ts

@@ -43,15 +43,21 @@ export async function proxy(request: NextRequest) {
       const port = process.env.PORT ?? "3000";
       const base = `http://localhost:${port}`;
 
+      const internalHeaders = {
+        "x-internal-secret": process.env.INTERNAL_SECRET ?? "",
+      };
+
       // Try slug first (D-06) — user-friendly slugs before token fallback
       let res = await fetch(
-        `${base}/api/internal/validate-slug?slug=${encodeURIComponent(slugOrToken)}`
+        `${base}/api/internal/validate-slug?slug=${encodeURIComponent(slugOrToken)}`,
+        { headers: internalHeaders }
       );
 
       // If slug not found, fall back to token validation (existing links continue to work)
       if (!res.ok) {
         res = await fetch(
-          `${base}/api/internal/validate-token?token=${encodeURIComponent(slugOrToken)}`
+          `${base}/api/internal/validate-token?token=${encodeURIComponent(slugOrToken)}`,
+          { headers: internalHeaders }
         );
       }