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/services/website_client.py

@@ -134,13 +134,17 @@ def normalize_groups(value: Any) -> list[dict[str, Any]]:
         name = item.get("name") or item.get("group_name") or str(gid)
         rate = item.get("rate_multiplier") or item.get("rateMultiplier") or item.get("ratio")
         desc = item.get("description") or item.get("desc") or item.get("remark")
-        groups.append({
+        platform = item.get("platform")
+        group = {
             "id": str(gid),
             "name": str(name),
             "rate_multiplier": fixed_decimal_string(rate, 2) if rate is not None else None,
             "description": str(desc) if desc is not None else None,
             "raw": item,
-        })
+        }
+        if platform is not None:
+            group["platform"] = str(platform)
+        groups.append(group)
     return groups