feat(sso): split SSO providers by delivery model across tiers (#754)

Custom OIDC stays on Community so self-hosters can wire any spec-compliant
OIDC identity provider (Authelia, Keycloak, Authentik, Zitadel, and others).
Google, GitHub, and Okta one-click presets move to Skipper. LDAP / Active
Directory and scoped RBAC are Admiral-only.

Backend enforces the split via a new requireTierForSsoProvider helper in
middleware/tierGates.ts, applied after requireAdmin in all four ssoConfig
mutation handlers. GET /sso/config (list) stays ungated so downgraded
admins can still see previously-configured providers. Invalid provider ids
now 400 before the tier check to avoid leaking tier information.

Frontend adds a compact mode to PaidGate and AdmiralGate for inline
list-item locks, and SSOSection reorders the provider cards as
Custom OIDC > Google > GitHub > Okta > LDAP to reinforce the
free-to-paid progression.

Stale 'SSO is Admiral' copy in AdmiralGate, PaidGate, and the Admiral
upgrade card on the License settings page has been replaced to reflect the
new split. User-facing licensing, SSO, overview, quickstart, and security
docs have been updated with the per-tier provider matrix.

docs/getting-started/sso-quickstart.mdx

@@ -5,6 +5,10 @@ description: Step-by-step instructions for connecting Sencho to your identity pr
 
 SSO can be configured via environment variables (shown below) or from the Settings UI after first boot.
 
+<Note>
+  **Tier availability.** Custom OIDC is available on every tier, including Community. The Google, GitHub, and Okta one-click presets require Skipper or higher. LDAP / Active Directory requires Admiral. See [Licensing & Billing](/features/licensing#feature-breakdown) for the full breakdown.
+</Note>
+
 ## Google OIDC
 
 1. Go to the [Google Cloud Console](https://console.cloud.google.com/apis/credentials)