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/scss/fusion_helpdesk.scss

@@ -170,3 +170,170 @@ $fhd-accent: var(--fhd-accent, $_fhd-accent-hex);
         &:hover { color: #d32f2f; }
     }
 }
+
+// Systray unread badge
+.o_fhd_systray {
+    .o_fhd_systray_btn { position: relative; }
+
+    .o_fhd_systray_badge {
+        position: absolute;
+        top: -2px;
+        right: 0;
+        min-width: 16px;
+        height: 16px;
+        padding: 0 4px;
+        border-radius: 8px;
+        background-color: #d9534f;
+        color: #fff;
+        font-size: 0.65rem;
+        font-weight: 700;
+        line-height: 16px;
+        text-align: center;
+    }
+}
+
+// Inbox additions (tabs, list, thread) — share the dialog tokens above.
+.o_fhd_dialog {
+    .o_fhd_muted { color: $fhd-muted; }
+
+    .o_fhd_tabs {
+        display: flex;
+        gap: 0.25rem;
+        border-bottom: 1px solid $fhd-border;
+        margin-bottom: 1rem;
+    }
+
+    .o_fhd_tab {
+        background: transparent;
+        border: none;
+        border-bottom: 2px solid transparent;
+        padding: 0.5rem 0.9rem;
+        color: $fhd-muted;
+        cursor: pointer;
+        font-weight: 500;
+
+        &:hover { color: $fhd-text; }
+
+        &.o_fhd_tab_active {
+            color: $fhd-accent;
+            border-bottom-color: $fhd-accent;
+        }
+    }
+
+    .o_fhd_scope_row {
+        display: flex;
+        gap: 0.5rem;
+        margin-bottom: 0.85rem;
+    }
+
+    // Ticket list
+    .o_fhd_ticket_list {
+        display: flex;
+        flex-direction: column;
+        border: 1px solid $fhd-border;
+        border-radius: 6px;
+        overflow: hidden;
+    }
+
+    .o_fhd_ticket_row {
+        display: flex;
+        align-items: center;
+        gap: 0.6rem;
+        padding: 0.6rem 0.75rem;
+        background-color: $fhd-bg;
+        border-bottom: 1px solid $fhd-border;
+        cursor: pointer;
+
+        &:last-child { border-bottom: none; }
+        &:hover { background-color: $fhd-hover; }
+    }
+
+    .o_fhd_unread_dot {
+        width: 9px;
+        height: 9px;
+        border-radius: 50%;
+        background-color: $fhd-accent;
+        flex: 0 0 auto;
+    }
+
+    .o_fhd_unread_spacer { width: 9px; flex: 0 0 auto; }
+
+    .o_fhd_ticket_ref {
+        color: $fhd-muted;
+        font-variant-numeric: tabular-nums;
+        flex: 0 0 auto;
+    }
+
+    .o_fhd_ticket_subject {
+        flex: 1;
+        white-space: nowrap;
+        overflow: hidden;
+        text-overflow: ellipsis;
+    }
+
+    .o_fhd_ticket_stage {
+        flex: 0 0 auto;
+        font-size: 0.78rem;
+        padding: 0.1rem 0.5rem;
+        border-radius: 10px;
+        background-color: $fhd-hover;
+        border: 1px solid $fhd-border;
+        color: $fhd-muted;
+    }
+
+    // Thread
+    .o_fhd_thread_head {
+        display: flex;
+        align-items: center;
+        justify-content: space-between;
+        margin-bottom: 0.6rem;
+    }
+
+    .o_fhd_open_portal {
+        font-size: 0.85rem;
+        color: $fhd-accent;
+        text-decoration: none;
+        &:hover { text-decoration: underline; }
+    }
+
+    .o_fhd_thread {
+        display: flex;
+        flex-direction: column;
+        gap: 0.6rem;
+        max-height: 45vh;
+        overflow-y: auto;
+        padding: 0.25rem;
+    }
+
+    .o_fhd_msg {
+        border: 1px solid $fhd-border;
+        border-radius: 6px;
+        padding: 0.6rem 0.75rem;
+        background-color: $fhd-bg;
+    }
+
+    .o_fhd_msg_head {
+        display: flex;
+        justify-content: space-between;
+        gap: 0.5rem;
+        margin-bottom: 0.3rem;
+        font-size: 0.82rem;
+    }
+
+    .o_fhd_msg_author { font-weight: 600; color: $fhd-text; }
+    .o_fhd_msg_date { color: $fhd-muted; font-variant-numeric: tabular-nums; }
+
+    .o_fhd_msg_body {
+        color: $fhd-text;
+        font-size: 0.9rem;
+        word-break: break-word;
+
+        p:last-child { margin-bottom: 0; }
+    }
+
+    .o_fhd_msg_attach {
+        margin-top: 0.4rem;
+        font-size: 0.8rem;
+        color: $fhd-muted;
+    }
+}