2.0.10: Add extension auto-update mechanism (check + 1-click update)

Detect newer plugin/template versions via the marketplace API and update
in one click, with a safety net — no cron/queue required (shared-host safe).

- ExtensionConfigDefault: non-abstract update(?string $fromVersion) hook,
  default success so existing format-1.0 plugins keep working unchanged.
- ExtensionUpdateManager: batch check with cache (no cron), download over
  TLS with checksum verify, filesystem backup + rollback, applyUpdate that
  overwrites files while leaving DB config intact.
- requireUpdateFrom gate: block unsafe major jumps (e.g. 1.x -> 2.9) with
  guidance; authoritative in the package manifest, mirrored from the server.
- ExtensionOnlineController: update() + checkUpdate() actions; local screen
  badge from cache. Routes, lang keys, update_check_ttl config.
- Scaffold: ship requireUpdateFrom placeholder + bump AppConfig format
  marker to 2.0; document the field in readme.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>