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>

main.go

@@ -21,6 +21,9 @@ func main() {
 	// 2. Initialize the database (auto-migrates, seeds admin).
 	db := models.InitDB(cfg)
 
+	// 2b. Warm the platform configuration cache from the database.
+	models.LoadConfigCache(db)
+
 	// 3. Create session store (cookie-based).
 	store := cookie.NewStore([]byte(cfg.Secret))
 	store.Options(sessions.Options{
@@ -65,6 +68,18 @@ func main() {
 		admin.POST("/articles/:id/delete", handlers.ArticleDelete(db))
 	}
 
+	// Protected admin settings routes (platform configuration).
+	settings := router.Group("/admin/settings")
+	settings.Use(middleware.AuthRequired())
+	{
+		settings.GET("/site", handlers.SiteSettingsPage(db))
+		settings.POST("/site", handlers.SiteSettingsSave(db, cfg.Path))
+		settings.GET("/upload", handlers.UploadSettingsPage(db))
+		settings.POST("/upload", handlers.UploadSettingsSave(db))
+		settings.GET("/download", handlers.DownloadSettingsPage(db))
+		settings.POST("/download", handlers.DownloadSettingsSave(db))
+	}
+
 	// Protected profile routes.
 	profile := router.Group("/profile")
 	profile.Use(middleware.AuthRequired())