docs(auth): clarify OAuth 2.1 server pricing is included in Auth MAUs (#49753)

OAuth 2.1 server had a single pricing statement anywhere, and it said
the feature is free during beta. This states the actual model everywhere
the feature is documented or sold: there is no separate charge, and
users who sign in through the OAuth server count toward Auth MAUs.

- docs getting started: replace the "free during beta" sentence with the
MAU-based pricing statement
- docs overview: add a Pricing section linking to the MAU usage guide
and the pricing page
- docs MCP authentication: note that agents authenticate as existing
users, and MAUs count per distinct user, so multiple agents for one user
count once
- www pricing comparison table: add an "OAuth 2.1 Server" row (included
on all plans) with a tooltip, and extend the MAU tooltip to cover OAuth
server sign-ins

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Clarified that OAuth 2.1 Server is available on all plans without a
separate charge.
* Explained that OAuth sign-ins count toward Monthly Active Users
(MAUs), with multiple agents for one user counted once.
  * Added links to MAU and pricing guidance.

* **Pricing**
* Added OAuth 2.1 Server as a plan feature and updated billing
descriptions for greater clarity.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/docs/content/guides/auth/oauth-server.mdx

@@ -19,6 +19,10 @@ There are several reasons why you might want to enable OAuth 2.1 Server in your
 
 - **Enterprise SSO**: Provide OpenID Connect (OIDC) authentication for enterprise customers who need standards-compliant identity federation across multiple services.
 
+## Pricing
+
+There is no separate charge for OAuth 2.1 Server. Users who sign in through your OAuth server count toward your project's [Monthly Active Users (MAUs)](/docs/guides/platform/manage-your-usage/monthly-active-users). See the [pricing page](/pricing) for the MAU quota on each plan.
+
 ## Overview
 
 Supabase Auth implements the OAuth 2.1 authorization code flow with PKCE (Proof Key for Code Exchange). When a third-party application wants to access user data:

apps/docs/content/guides/auth/oauth-server/getting-started.mdx

@@ -31,7 +31,7 @@ Testing OAuth flows is often easier on a Supabase project since it's already acc
 
 ## Enable OAuth 2.1 server
 
-OAuth 2.1 server is currently in beta and free to use during the beta period on all Supabase plans.
+OAuth 2.1 server is in beta and available on all Supabase plans. It has no separate charge. Users who sign in through your OAuth server count toward your project's [Monthly Active Users (MAUs)](/docs/guides/platform/manage-your-usage/monthly-active-users).
 
 <Tabs
   scrollable

apps/docs/content/guides/auth/oauth-server/mcp-authentication.mdx

@@ -30,6 +30,12 @@ When you build an MCP server that connects to your Supabase project, authenticat
 
 With Supabase Auth, your MCP server can authenticate AI agents using your existing user accounts without building a separate authentication system.
 
+<Admonition type="note">
+
+MCP authentication has no separate charge. AI agents authenticate as your existing users, so their sign-ins count toward your project's [Monthly Active Users (MAUs)](/docs/guides/platform/manage-your-usage/monthly-active-users). Supabase counts MAUs per distinct user, so multiple agents or MCP clients acting for the same user count as one MAU.
+
+</Admonition>
+
 ## Prerequisites
 
 Before setting up MCP authentication: