enable_plugin() now auto-installs available plugins before enabling them.
This fixes the UX issue where clicking Enable on an AVAILABLE plugin would
fail with 'Plugin is not installed'. Now it installs automatically (runs
migrations, seeds permissions) then enables it.
Before: User had to manually install via CLI first
After: Enable button works on AVAILABLE plugins (one-click install+enable)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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