- ISessionStore: all methods use (platform, external_user_id)
- SessionService: updated SQL queries and added UpsertDiscordUserAsync
- AuthorizedSessionService: resolves identity from HttpContext, no longer accepts telegram_id params
- SessionAccessDeniedException now accepts string externalUserId
- Added ExternalUserId/ExternalUsername to WebGroupManager and WebParticipant
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>