feat: backend support for creating and storing service accounts (#22698)

Add is_service_account column to users table with CHECK constraints
enforcing login_type='none' and empty email for service accounts.
Update user creation API to validate service account constraints.

Related to:
https://linear.app/codercom/issue/PLAT-27/feat-backend-support-for-creating-and-storing-service-accounts

site/src/api/typesGenerated.ts

@@ -2076,6 +2076,10 @@ export interface CreateUserRequestWithOrgs {
 	 * OrganizationIDs is a list of organization IDs that the user should be a member of.
 	 */
 	readonly organization_ids: readonly string[];
+	/**
+	 * Service accounts are admin-managed accounts that cannot login.
+	 */
+	readonly service_account?: boolean;
 }
 
 // From codersdk/workspaces.go