feat: add platform configuration tables and admin settings

Add four DB-backed configuration tables for site-wide settings:
site_settings (logo, top-left title, header/footer text with zh/en
variants), upload_configs (master switch, default size, storage dir),
upload_file_types (per-extension whitelist with per-type size limits),
and download_baseurls (multiple download sources with priority/default).

- Models, AutoMigrate, and first-run seed (18 common file types)
- Process-level config cache warmed at startup, refreshed on admin save
- SetUserContext injects cached site settings into templates
- base.html renders logo (local upload or external URL), title, header
  banner, and footer with i18n fallback
- Upload validator drives avatar uploads (form + AJAX) through the
  configured switch/type/size policy
- Admin pages for site/upload/download settings with i18n keys

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

templates/admin/dashboard.html

@@ -50,6 +50,10 @@
                 class="inline-flex items-center gap-2 bg-gray-200 text-gray-700 px-6 py-3 rounded-lg font-medium hover:bg-gray-300 transition-colors">
                 {{index .Tr "article_manage"}}
             </a>
+            <a href="/admin/settings/site"
+                class="inline-flex items-center gap-2 bg-gray-200 text-gray-700 px-6 py-3 rounded-lg font-medium hover:bg-gray-300 transition-colors">
+                {{index .Tr "settings_nav"}}
+            </a>
         </div>
     </div>
 </section>