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/schemas/website.py

@@ -65,6 +65,7 @@ class WebsiteGroupResponse(BaseModel):
     name: str
     rate_multiplier: Optional[str] = None
     description: Optional[str] = None
+    platform: Optional[str] = None
     raw: dict[str, Any] = {}
 
 
@@ -115,6 +116,7 @@ class BindingResponse(BaseModel):
     percent: float
     algorithm: str
     enabled: bool
+    platform: Optional[str] = None
     created_at: datetime
     updated_at: datetime