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/__manifest__.py

@@ -3,7 +3,7 @@
 # License OPL-1 (Odoo Proprietary License v1.0)
 {
     'name': 'Fusion Helpdesk Reporter',
-    'version': '19.0.2.0.0',
+    'version': '19.0.2.1.0',
     'category': 'Productivity',
     'summary': 'One-click in-app bug reporting & feature requesting — '
                'auto-creates a helpdesk.ticket on a central Odoo Helpdesk.',