fix: keep the other account out of the OIDC link conflict

Linking a provider identity already bound elsewhere answered 409 with the raw
provider subject and the other account's internal id in the message. The caller
needs to know there is a conflict, not whose it is; both stay in the log for
whoever is debugging it.

Status and legacy code are unchanged — OIDCService branches on the 409.

src/backend/stores/oidc/OIDCStore.js

@@ -76,9 +76,16 @@ export class OIDCStore extends PuterStore {
         const existing = await this.getByProviderSub(provider, providerSub);
         if (!existing) return;
         if (existing.user_id !== userId) {
+            // The caller is told there is a conflict, not whose.
+            console.warn('[oidc] link conflict', {
+                provider,
+                providerSub,
+                boundTo: existing.user_id,
+                userId,
+            });
             throw new HttpError(
                 409,
-                `OIDC link conflict: (${provider}, ${providerSub}) already bound to user ${existing.user_id}`,
+                `This ${provider} identity is already linked to another account`,
                 { legacyCode: 'conflict' },
             );
         }

src/backend/stores/oidc/OIDCStore.test.ts

@@ -19,7 +19,10 @@ const createPostgresUniqueError = (): Error & { code: string } => {
 const createStore = (rows: readonly OidcRow[]) => {
     const db = {
         write: vi.fn(
-            async (_sql: string, _params: readonly unknown[]): Promise<void> => {
+            async (
+                _sql: string,
+                _params: readonly unknown[],
+            ): Promise<void> => {
                 throw createPostgresUniqueError();
             },
         ),
@@ -70,5 +73,12 @@ describe('OIDCStore', () => {
             statusCode: 409,
             legacyCode: 'conflict',
         });
+
+        // Neither the other account's id nor the raw provider subject.
+        const error = await store
+            .link(123, 'test-provider', 'subject-1')
+            .catch((e: Error) => e);
+        expect(error.message).not.toContain('456');
+        expect(error.message).not.toContain('subject-1');
     });
 });