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>

.env.example

@@ -1,2 +1,12 @@
 # Database — Postgres su Coolify
 DATABASE_URL=postgresql://user:password@host:5432/database
+
+# NextAuth
+NEXTAUTH_URL=https://hub.iamcavalli.net
+NEXTAUTH_SECRET=generate-with-openssl-rand-base64-32
+ADMIN_EMAIL=admin@example.com
+ADMIN_PASSWORD=use-a-strong-password-min-20-chars
+
+# Internal API secret — shared between proxy.ts and /api/internal/* routes
+# Generate with: openssl rand -base64 32
+INTERNAL_SECRET=generate-with-openssl-rand-base64-32