Implement security hardening across frontend, backend, and infrastructure

- nginx: add X-Content-Type-Options, X-Frame-Options, X-XSS-Protection,
  and Referrer-Policy headers on all responses; rate limit /api/auth/login
  to 5 req/min per IP (burst 3) to prevent brute force
- frontend: add escHtml() utility to api.js; use it on all notes fields
  across dashboard, log, history, flock, and budget pages to prevent XSS
- log.js: fix broken loadRecent() call referencing removed #recent-body
  element; replaced with loadHistory() from history.js
- schemas.py: raise minimum password length from 6 to 10 characters
- admin.py: add audit logging for password reset, disable, delete, and
  impersonate actions; fix impersonate to use named admin param for logging
- main.py: add startup env validation — exits with clear error if any
  required env var is missing; configure structured logging to stdout
- docker-compose.yml: add log rotation (10 MB / 3 files) to all services

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

nginx/html/js/dashboard.js

@@ -115,7 +115,7 @@ async function loadDashboard() {
             <tr>
                 <td>${fmtDate(e.date)}</td>
                 <td>${e.eggs}</td>
-                <td class="notes">${e.notes || ''}</td>
+                <td class="notes">${escHtml(e.notes)}</td>
             </tr>
         `).join('');