refactor(fusion_helpdesk): owner contact is a res.partner, not two text fields

Smaller UX simplification on the client side: the owner is already a
contact in entech's address book, so picking one is faster + safer than
re-typing their email and name (and avoids typos creeping into the
approval-email To: header).

What changed:
- Entech settings: drop fhd_owner_email + fhd_owner_name char fields;
  add fhd_owner_partner_id Many2one to res.partner exposed in the
  same "Owner Approval" block as a single partner selector. Quick-create
  + create-and-edit kept enabled so admins can spin up a new partner
  inline if the owner isn't already in the system.
- controllers/main.py::_read_config: derives owner_email + owner_name
  from the selected partner via the new _resolve_owner_contact helper.
  Missing / dangling partner id → blank email + name → central simply
  won't see the keys and the Engage button stays disabled (correct
  "not configured" behaviour).
- Nexa side: ZERO changes. Still receives owner_email + owner_name
  strings on the ticket payload, still upserts client_key.owner_email/
  name. The partner abstraction stops at the entech boundary.
- migrations/19.0.2.1.0/post-migration.py auto-resolves the legacy
  fusion_helpdesk.owner_email ICP value to an existing res.partner
  (lowest-id match on lowercased email), writes the new
  fusion_helpdesk.owner_partner_id key, and deletes the obsolete
  owner_email + owner_name ICP rows so a future reader doesn't trip
  over stale config.

Verified live on entech: kris@enplating.ca → res.partner #2308 ("Kris
Pathinather"), legacy keys purged, controller._resolve_owner_contact
returns the expected (email, name). The piggyback payload is unchanged
so existing client_key sync continues to work without a central
redeploy.

Bumps fusion_helpdesk to 19.0.2.1.0. fusion_helpdesk_central stays at
19.0.2.0.0 (no central-side changes required).

fusion_helpdesk/models/res_config_settings.py

@@ -50,22 +50,21 @@ class ResConfigSettings(models.TransientModel):
              'can tell which client deployment a ticket came from. '
              'e.g. "ENTECH" → "[ENTECH] My subject"',
     )
-    # Owner contact for the central engagement / approval flow. Optional —
-    # leaving these blank disables the "Request Owner Approval" button on
-    # the central side for this client. Both values piggyback on every
-    # ticket submission (see controllers/main.py::submit) so central always
-    # has the latest contact without a dedicated sync endpoint.
-    fhd_owner_email = fields.Char(
-        string='Owner Email',
-        config_parameter='fusion_helpdesk.owner_email',
-        help='Email of the real decision-maker at your company — the '
-             'person who can approve feature requests or bug-fix scope. '
-             'Used when central support hits a ticket that needs sign-off. '
+    # Owner contact for the central engagement / approval flow. Picked
+    # from the existing res.partner — the owner is already a contact in
+    # the system, no point retyping their email and name. Email + name
+    # are derived from the partner at submit-time and piggybacked on
+    # every ticket payload (see controllers/main.py::submit) so central
+    # always has the latest contact without a dedicated sync endpoint.
+    # Leaving the partner blank disables the "Request Owner Approval"
+    # button on the central side for this client.
+    fhd_owner_partner_id = fields.Many2one(
+        'res.partner',
+        string='Owner Contact',
+        config_parameter='fusion_helpdesk.owner_partner_id',
+        help='The real decision-maker at your company — the person who '
+             'can approve feature requests or bug-fix scope. Used when '
+             'central support hits a ticket that needs sign-off. '
+             'Email and name are taken from the selected contact. '
              'Leave blank if your deployment doesn\'t require approvals.',
     )
-    fhd_owner_name = fields.Char(
-        string='Owner Name',
-        config_parameter='fusion_helpdesk.owner_name',
-        help='Display name for the owner — shown in the approval email '
-             'greeting and in the chatter attribution after they decide.',
-    )