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/app/routers/upstreams.py

@@ -15,7 +15,7 @@ from sqlalchemy.orm import Session
 from app.database import SessionLocal, get_db
 from app.models.admin_user import AdminUser
 from app.models.upstream import Upstream
-from app.models.upstream_key import UpstreamGeneratedKey
+from app.models.upstream_key import UpstreamGeneratedKey, UpstreamKeyAccountLink
 from app.models.upstream_recharge_event import UpstreamRechargeEvent
 from app.models.snapshot import UpstreamRateSnapshot
 from app.schemas.upstream import (
@@ -25,6 +25,7 @@ from app.schemas.upstream import (
     UpstreamCreate, UpstreamUpdate, UpstreamResponse, SnapshotResponse, TestResult,
     UpstreamBatchActionItem, UpstreamBatchActionSummary, UpstreamBatchActionResponse,
     UpstreamRechargeCreate, UpstreamRechargeUpdate, UpstreamRechargeResponse,
+    UpstreamKeyAccountLinkResponse,
 )
 from app.services.upstream_client import UpstreamClient, UpstreamError, _PendingKeyError, build_snapshot, build_new_api_token_name, mask_secret, _extract_key_value
 from app.services.auth_config import MASK, mask_auth_config, normalize_auth_config
@@ -49,7 +50,13 @@ def _group_name(group: dict, gid: str) -> str:
     return str(group.get("name") or group.get("group_name") or gid)
 
 
-def _key_response(row: UpstreamGeneratedKey, include_value: bool = False) -> GeneratedUpstreamKeyResponse:
+def _key_response(row: UpstreamGeneratedKey, include_value: bool = False, db: Session | None = None) -> GeneratedUpstreamKeyResponse:
+    sess = db or Session.object_session(row)
+    imported_accounts = []
+    if sess and row.id is not None:
+        links = sess.query(UpstreamKeyAccountLink).filter(UpstreamKeyAccountLink.upstream_key_id == row.id).all()
+        imported_accounts = [UpstreamKeyAccountLinkResponse.model_validate(l) for l in links]
+
     return GeneratedUpstreamKeyResponse(
         id=row.id,
         upstream_id=row.upstream_id,
@@ -67,6 +74,7 @@ def _key_response(row: UpstreamGeneratedKey, include_value: bool = False) -> Gen
         created_at=row.created_at,
         updated_at=row.updated_at,
         has_key_value=bool(row.key_value),
+        imported_accounts=imported_accounts,
     )