fix(oidc): preserve authProvider on auto-link

Linked users keep authProvider="local" so they can still log in
with their local password. Only externalId is set during linking.

apps/api/src/plugins/oidc.ts

@@ -309,7 +309,6 @@ export async function oidcRoutes(app: FastifyInstance): Promise<void> {
         db.update(schema.users)
           .set({
             externalId: sub,
-            authProvider: "oidc",
             updatedAt: new Date(),
           })
           .where(eq(schema.users.id, existingByEmail.id))