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Squash-merge of feat/helpdesk-customer-followup. The billing and fusion_login_audit work from that branch is already on main (landed separately); this lands only the helpdesk feature. - Identity keystone: submit() forwards partner_email/partner_name/ x_fc_client_label so the central Helpdesk find-or-creates the customer partner and subscribes them as a follower (enables reply emails + magic link). - Embedded in-app 'My Tickets' inbox: server-side scoped read/reply RPC endpoints, per-user seen tracking (fusion.helpdesk.ticket.seen), systray unread badge. Defense-in-depth scope domain + _norm_email normalisation (wildcard emails cannot widen scope). - fusion_helpdesk_central: x_fc_client_label field + list/search views + branded acknowledgement email template. - Deployed and smoke-tested live: nexa central 19.0.1.1.0, entech client 19.0.1.4.1 (requires Contact Creation on the central service account). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>