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>

middleware/auth.go

@@ -95,6 +95,16 @@ func SetUserContext(db *gorm.DB) gin.HandlerFunc {
 		c.Set("avatar", avatar)
 		c.Set("display_name", displayName)
 		c.Set("role", role)
+
+		// --- Site platform configuration (from DB cache) ---
+		site := models.GetSiteSetting()
+		c.Set("site_setting", site)
+		c.Set("site_logo", site.Logo)
+		c.Set("site_logo_is_url", site.LogoIsURL())
+		c.Set("site_logo_text", site.LogoText(string(lang)))
+		c.Set("site_header_text", site.HeaderText(string(lang)))
+		c.Set("site_footer_text", site.FooterText(string(lang)))
+
 		c.Next()
 	}
 }