feat(ux): backend management enhancement (#9325)

* feat: add PreferDevelopmentBackends setting, expose isMeta/isDevelopment in API

- Add PreferDevelopmentBackends config field, CLI flag, runtime setting
- Add IsDevelopment() method to GalleryBackend
- Use AvailableBackendsUnfiltered in UI API to show all backends
- Expose isMeta, isDevelopment, preferDevelopmentBackends in backend API response

* feat: upgrade banner with Upgrade All button, detect pre-existing backends

- Add upgrade banner on Backends page showing count and Upgrade All button
- Fix upgrade detection for backends installed before version tracking:
  flag as upgradeable when gallery has a version but installed has none
- Fix OCI digest check to flag backends with no stored digest as upgradeable