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>

Dockerfile

@@ -15,7 +15,7 @@ RUN npm run build
 FROM base AS runner
 WORKDIR /app
 ENV NODE_ENV=production
-ENV NEXTAUTH_URL=https://hub.iamcavalli.net
+# NEXTAUTH_URL must be set via Coolify environment variables at runtime
 RUN addgroup --system --gid 1001 nodejs
 RUN adduser --system --uid 1001 nextjs
 COPY --from=builder /app/public ./public