feat(fusion_helpdesk_central): expose OpenAI key + cron settings in UI

Adding the 'Fusion Helpdesk Central' block to General Settings so the
three ICP keys the engagement flow reads are configurable from a real
form instead of forcing admins to open Technical → System Parameters.

Three settings, all wired via config_parameter= so the existing read
paths (engagement_wizard, _fc_send_engagement_reminders) keep working
unchanged:

- fusion_helpdesk_central.openai_api_key (password widget — doesn't
  render plaintext on the form)
- fusion_helpdesk_central.openai_model (default 'gpt-4o-mini')
- fusion_helpdesk_central.engagement_reminder_days (default 3, 0
  disables the reminder cron entirely)

Bumps fusion_helpdesk_central to 19.0.2.2.0.

Find under Settings → Fusion Helpdesk Central. The block has two
sub-sections: "Owner Approval — AI Summary" (key + model) and
"Owner Approval — Reminder Cadence" (days).

fusion_helpdesk_central/models/__init__.py

@@ -2,3 +2,4 @@
 from . import fusion_helpdesk_client_key
 from . import helpdesk_ticket
 from . import engagement_wizard
+from . import res_config_settings

fusion_helpdesk_central/models/res_config_settings.py

@@ -0,0 +1,45 @@
+# -*- coding: utf-8 -*-
+# Copyright 2026 Nexa Systems Inc.
+# License OPL-1
+"""Settings for fusion_helpdesk_central exposed via General Settings.
+
+Surfaces the three ICP keys the engagement flow reads — OpenAI key /
+model / reminder cadence — so admins don't have to dig into Technical →
+System Parameters to set up the AI summary or change the reminder
+window. All three are stored in ir.config_parameter via the
+`config_parameter=` shortcut so the existing read paths
+(engagement_wizard, helpdesk_ticket._fc_send_engagement_reminders)
+continue to work unchanged.
+"""
+from odoo import fields, models
+
+
+class ResConfigSettings(models.TransientModel):
+    _inherit = 'res.config.settings'
+
+    fhc_openai_api_key = fields.Char(
+        string='OpenAI API Key',
+        config_parameter='fusion_helpdesk_central.openai_api_key',
+        help='Server-side key the engagement wizard uses to generate AI '
+             'summaries on Engage. Leave blank to disable AI summaries — '
+             'the wizard still works, you just write the brief manually. '
+             'Keep secret: anyone with access to System Parameters could '
+             'read it back.',
+    )
+    fhc_openai_model = fields.Char(
+        string='OpenAI Model',
+        config_parameter='fusion_helpdesk_central.openai_model',
+        default='gpt-4o-mini',
+        help='OpenAI model used for the engagement summary. Default '
+             'gpt-4o-mini balances cost and quality — switch to gpt-4o '
+             'or gpt-4o-2024-08-06 if you find summaries too thin.',
+    )
+    fhc_engagement_reminder_days = fields.Integer(
+        string='Reminder After (days)',
+        config_parameter='fusion_helpdesk_central.engagement_reminder_days',
+        default=3,
+        help='Days of inaction before the cron sends a single follow-up '
+             'reminder to the owner. Set to 0 to disable reminders '
+             'entirely. The cron sends at most one reminder per '
+             'engagement regardless of this value.',
+    )