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>

models/db.go

@@ -45,10 +45,15 @@ func InitDB(cfg *config.Config) *gorm.DB {
 	}
 
 	// Auto-migrate tables (idempotent).
-	if err := db.AutoMigrate(&User{}, &Article{}); err != nil {
+	if err := db.AutoMigrate(&User{}, &Article{}, &SiteSetting{}, &UploadConfig{}, &UploadFileType{}, &DownloadBaseURL{}); err != nil {
 		log.Fatalf("Failed to auto-migrate database: %v", err)
 	}
 
+	// Seed site platform configuration on first run.
+	seedSiteSettings(db)
+	seedUploadConfig(db)
+	seedUploadFileTypes(db)
+
 	// First-run seed: create admin user if no users exist.
 	var count int64
 	db.Model(&User{}).Count(&count)