feat: track imported accounts per upstream key across platforms

Add upstream_key_account_links table mapping generated keys to remote
accounts per website/platform, surface imported_accounts on key
responses, and update website sync/routers to manage the links.

backend/test_group_binding_create_sync.py

@@ -137,9 +137,17 @@ def test_create_binding_runs_initial_sync(monkeypatch, db_session):
     assert log.new_rate == "2.20"
 
 
-def test_create_binding_skips_write_when_website_auto_sync_disabled(db_session):
+def test_create_binding_skips_write_when_website_auto_sync_disabled(monkeypatch, db_session):
     website, upstream = seed_rows(db_session, auto_sync_enabled=False)
 
+    class FakeClient:
+        def __init__(self, **kwargs): pass
+        def __enter__(self): return self
+        def __exit__(self, *args): pass
+        def get_groups(self, endpoint):
+            return [{"id": "target", "name": "Target group", "platform": "openai"}]
+    monkeypatch.setattr(websites_router, "Sub2ApiWebsiteClient", FakeClient)
+
     result = websites_router.create_binding(
         make_body(website.id, upstream.id),
         db_session,
@@ -155,9 +163,17 @@ def test_create_binding_skips_write_when_website_auto_sync_disabled(db_session):
     assert log.new_rate == "2.20"
 
 
-def test_create_binding_skips_write_when_binding_disabled(db_session):
+def test_create_binding_skips_write_when_binding_disabled(monkeypatch, db_session):
     website, upstream = seed_rows(db_session)
 
+    class FakeClient:
+        def __init__(self, **kwargs): pass
+        def __enter__(self): return self
+        def __exit__(self, *args): pass
+        def get_groups(self, endpoint):
+            return [{"id": "target", "name": "Target group", "platform": "openai"}]
+    monkeypatch.setattr(websites_router, "Sub2ApiWebsiteClient", FakeClient)
+
     result = websites_router.create_binding(
         make_body(website.id, upstream.id, enabled=False),
         db_session,
@@ -171,8 +187,15 @@ def test_create_binding_skips_write_when_binding_disabled(db_session):
     assert log.new_rate == "2.20"
 
 
-def test_create_binding_keeps_binding_when_initial_sync_calculation_fails(db_session):
+def test_create_binding_keeps_binding_when_initial_sync_calculation_fails(monkeypatch, db_session):
     website, upstream = seed_rows(db_session)
+    class FakeClient:
+        def __init__(self, **kwargs): pass
+        def __enter__(self): return self
+        def __exit__(self, *args): pass
+        def get_groups(self, endpoint):
+            return [{"id": "target", "name": "Target group", "platform": "openai"}]
+    monkeypatch.setattr(websites_router, "Sub2ApiWebsiteClient", FakeClient)
     db_session.query(UpstreamRateSnapshot).delete()
     db_session.commit()