feat: edit UI configuration from the admin page

[JeromeBu]

Summary

• persist UI configuration as a singleton JSONB document in PostgreSQL and keep the existing admin editor
• provide a lightweight live JSON syntax highlight in the editor without adding a client dependency
• import a mounted legacy ui-config.json exactly once from the config_ui migration, or use an embedded standard configuration for new installations
• validate migration input against the frozen historical schema and fail explicitly on unreadable, malformed, or incompatible files
• require a valid singleton row at startup and update it without implicit recreation
• bootstrap the first Catalogi administrator only when the OIDC email matches CATALOGI_INITIAL_ADMIN_EMAIL and no administrator exists, including concurrent callbacks and case-insensitive lookup
• document fresh installation and upgrade workflows for Docker Compose and Helm
• allow the one-shot Helm compatibility mount to be disabled independently while retaining translation customization

Validation

• 27 targeted migration, UI configuration, deployment example, and authentication tests pass
• 132 API tests pass and 4 are skipped when the two live GitHub-dependent test files are excluded
• API/web/e2e typechecks, formatting, and web lint pass
• API and web production builds pass
• Helm renders successfully with the compatibility import enabled, disabled while translations remain enabled, all customization disabled, and with every checked-in example values file
• the two live GitHub-dependent test files could not be validated locally because GitHub returned HTTP 403/rate-limit responses; they remain covered by CI

Deployment notes

For an existing deployment, keep the legacy ui-config.json mount for the first startup on this version, verify the imported value, then remove the mount. For Helm, set customization.legacyUiConfigImportEnabled to false after the successful import. PostgreSQL is the only source of truth afterwards.

For a new deployment, configure OIDC and set CATALOGI_INITIAL_ADMIN_EMAIL to the intended first administrator. After that address has signed in successfully, the variable can be removed. Catalogi creates no local password or admin/admin account.

Deliberate scope

This PR does not add administrator role management in the UI. It keeps last-write-wins behavior for concurrent UI configuration edits and does not add configuration history.