feat(fusion_helpdesk): customer follow-up + embedded ticket inbox

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>

fusion_helpdesk/static/src/xml/fusion_helpdesk_systray.xml

@@ -5,11 +5,14 @@
         <div class="o_fhd_systray dropdown">
             <button type="button"
                     class="o_fhd_systray_btn dropdown-toggle"
-                    title="Report a bug or request a feature"
+                    title="Report a bug, request a feature, or follow up on your tickets"
                     t-on-click="onClick">
                 <img src="/fusion_helpdesk/static/description/help_icon.png"
                      alt="Help"
                      class="o_fhd_systray_img"/>
+                <span t-if="state.unread > 0"
+                      class="o_fhd_systray_badge"
+                      t-esc="state.unread > 99 ? '99+' : state.unread"/>
             </button>
         </div>
     </t>