Fall back to unsigned.prev_sender for auto-joined rooms' key bundles
An accepted knock can be auto-joined straight past the invited state
(the invite coalesced out of sync), and the invite-time record is
in-memory only, so recover the inviter from the join event's
unsigned.prev_sender — the same Synapse extension getDMInviter already
relies on. Mirrors the equivalent rust-sdk fix.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uk8aPxHn3BHCe52L226jdG