The persisted (DB-fallback) branch of _resume_target_allowed() compared only
sessions.user_id against source.user_id, but build_session_key() keys the
participant on `user_id_alt or user_id` (Signal/Feishu carry the canonical
participant in user_id_alt). The sessions table has no user_id_alt column, so a
per-user row a caller shares the user_id of — but not the user_id_alt — maps to a
DIFFERENT live session key, yet the row's user_id matched both participants:
a co-member could resume/enumerate another member's persisted per-user group or
no-chat_id DM session (IDOR, CWE-639).
The live-origin guard (_same_origin_chat) already compares user_id_alt; the
persisted fallback couldn't. Fail closed on both identity-bearing per-user
branches (non-DM per-user group, no-chat_id DM) whenever the caller carries a
user_id_alt. Shared group/thread sessions (no participant scoping) and DMs keyed
on a present chat_id are unaffected; callers keyed on user_id (e.g. Telegram)
still resume their own rows; admin --all override still applies.
Regression: tests/gateway/test_resume_command.py::
test_resume_persisted_fallback_fails_closed_on_user_id_alt.
5b3f064259