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>
