feat(fusion_repairs): Phase 3 - maintenance contracts + client self-booking

Maintenance contracts
- New fusion.repair.maintenance.contract model: one per partner +
  product + lot. Fields: interval_months, last_service_date,
  next_due_date, state, booking_token (secrets.token_urlsafe),
  last_reminder_band (30 / 7 / 1), booking_repair_id
- roll_next_due_date() advances the cycle by interval_months and resets
  the band / booked-repair so the next cycle starts fresh
- sale.order._spawn_maintenance_contracts() creates contracts for
  delivered SOs whose product has x_fc_maintenance_interval_months > 0
  (called from Phase 3 hooks; ready for cron / on-state change wiring)

Reminder cron
- Daily ir.cron at 07:00 -> cron_send_due_reminders()
- Sends email at 30 / 7 / 1 day bands before next_due_date; tracks
  last_reminder_band so we never re-send the same band in one cycle
- Master toggle via ir.config_parameter fusion_repairs.enable_email_notifications

Public client booking portal
- /repairs/maintenance/book/<token>  GET landing page with a date input
- /repairs/maintenance/book/<token>/confirm  POST creates a repair.order
  via contract.create_repair_from_booking() (source='client_portal')
- Idempotent: existing booking shows "already booked" instead of
  spawning a duplicate
- Invalid / expired tokens render a friendly "link not valid" page

Mail template
- email_template_maintenance_due_reminder with 4px green accent bar,
  600px max-width, dark/light safe; renders the tokenized booking CTA
  button directly to /repairs/maintenance/book/<token>

Backend
- Maintenance Contracts list / form with statusbar + chatter
- Menu under Operations -> Maintenance Contracts
- Sequence MC/##### for contract reference
- Access rules: User read, Dispatcher write, Manager full

Verified end-to-end on local westin-v19:
- Contract MC/00003 created due in 7 days
- cron_send_due_reminders() fires the 7-day band; second invocation
  skips (idempotent)
- create_repair_from_booking() spawns BR-WA/RO/00014 with
  x_fc_intake_source='client_portal' and links it back to the contract
- HTTP GET /repairs/maintenance/book/<token> -> 200 with the date input
  and contract reference visible in the page

Co-authored-by: Cursor <cursoragent@cursor.com>

fusion_repairs/controllers/__init__.py

@@ -4,3 +4,4 @@
 
 from . import portal_sales_rep_repair
 from . import portal_client_repair
+from . import portal_maintenance_booking

fusion_repairs/controllers/portal_maintenance_booking.py

@@ -0,0 +1,70 @@
+# -*- coding: utf-8 -*-
+# Copyright 2024-2026 Nexa Systems Inc.
+# License OPL-1 (Odoo Proprietary License v1.0)
+
+"""Client maintenance booking portal.
+
+The maintenance reminder email contains a tokenized URL:
+  /repairs/maintenance/book/<token>
+
+Clicking it lands the client on a single-page form where they can confirm
+a preferred date. On submit, a repair.order is spawned via the same
+intake service (source='client_portal') and the contract's next reminder
+band is locked so we don't keep nagging them.
+"""
+
+import logging
+
+from odoo import _, fields, http
+from odoo.http import request
+
+_logger = logging.getLogger(__name__)
+
+
+class MaintenanceBookingPortal(http.Controller):
+
+    def _resolve_contract(self, token):
+        if not token:
+            return None
+        Contract = request.env['fusion.repair.maintenance.contract'].sudo()
+        contract = Contract.search([('booking_token', '=', token)], limit=1)
+        if not contract or contract.state != 'active':
+            return None
+        return contract
+
+    @http.route('/repairs/maintenance/book/<string:token>', type='http',
+                auth='public', website=True, sitemap=False)
+    def maintenance_book_get(self, token, **kw):
+        contract = self._resolve_contract(token)
+        if not contract:
+            return request.render('fusion_repairs.portal_maintenance_invalid_token', {})
+        already = bool(contract.booking_repair_id)
+        return request.render('fusion_repairs.portal_maintenance_book', {
+            'contract': contract,
+            'already_booked': already,
+            'default_date': fields.Date.context_today(request.env.user).isoformat(),
+        })
+
+    @http.route('/repairs/maintenance/book/<string:token>/confirm', type='http',
+                auth='public', methods=['POST'], csrf=True, website=True)
+    def maintenance_book_post(self, token, **post):
+        contract = self._resolve_contract(token)
+        if not contract:
+            return request.render('fusion_repairs.portal_maintenance_invalid_token', {})
+
+        if contract.booking_repair_id:
+            return request.redirect(f'/repairs/maintenance/book/{token}?ok=already')
+
+        preferred_date = (post.get('preferred_date') or '').strip()
+        scheduled = False
+        if preferred_date:
+            try:
+                scheduled = fields.Date.from_string(preferred_date)
+            except ValueError:
+                scheduled = False
+
+        repair = contract.create_repair_from_booking(scheduled_date=scheduled)
+        return request.render('fusion_repairs.portal_maintenance_thanks', {
+            'contract': contract,
+            'repair': repair,
+        })