Providers start disabled until models are available

What changed

New and preset providers (except CherryAI) are now created and seeded disabled. They auto-enable once usable models are discovered through the existing provider-settings flows (auto model sync, pull reconcile, or a successful connection check).

Why this matters to the user

On a fresh install, most providers appear disabled in Settings → Providers until a model list has been fetched. They remain visible because the provider sidebar now defaults to showing all providers. Existing installs are unaffected — the migration preserves each provider's current enabled state.

What the user should do

Nothing — automatic. Adding an API key (or otherwise triggering model discovery) enables the provider; a provider can also be toggled on manually from the sidebar.