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/layouts/base.html

@@ -15,8 +15,15 @@
     <!-- Navigation -->
     <nav class="bg-white shadow-sm border-b border-gray-200">
         <div class="max-w-5xl mx-auto px-4 py-3 flex items-center justify-between">
-            <a href="/" class="text-xl font-bold text-gray-800 hover:text-blue-600 transition-colors">
-                {{index .Tr "site_title"}}
+            <a href="/" class="text-xl font-bold text-gray-800 hover:text-blue-600 transition-colors flex items-center gap-2">
+                {{if .SiteLogo}}
+                    {{if .SiteLogoIsURL}}
+                        <img src="{{.SiteLogo}}" alt="logo" class="h-8 w-auto">
+                    {{else}}
+                        <img src="/uploads/logos/{{.SiteLogo}}" alt="logo" class="h-8 w-auto">
+                    {{end}}
+                {{end}}
+                {{if .SiteLogoText}}{{.SiteLogoText}}{{else}}{{index .Tr "site_title"}}{{end}}
             </a>
             <div class="flex items-center gap-4">
                 <a href="/" class="text-gray-600 hover:text-blue-600 transition-colors">{{index .Tr "home"}}</a>
@@ -62,6 +69,12 @@
 
     <!-- Main Content -->
     <main class="flex-1">
+    {{if .SiteHeaderText}}
+        <!-- Header banner -->
+        <div class="bg-blue-50 border-b border-blue-100 text-blue-800 text-sm text-center py-2 px-4">
+            {{.SiteHeaderText}}
+        </div>
+    {{end}}
 {{end}}
 
 {{define "footer"}}
@@ -70,7 +83,7 @@
     <!-- Footer -->
     <footer class="bg-white border-t border-gray-200 py-6 mt-auto">
         <div class="max-w-5xl mx-auto px-4 text-center text-gray-500 text-sm">
-            {{index .Tr "footer_text"}}
+            {{if .SiteFooterText}}{{.SiteFooterText}}{{else}}{{index .Tr "footer_text"}}{{end}}
         </div>
     </footer>
     <script>