feat(billing): design + scaffold fusion_centralize_billing

Centralize billing for all NexaSystems services (NexaCloud, NexaDesk,
NexaMaps, custom apps, memberships) on the Odoo 19 Enterprise instance,
replacing Lago. The module adds only the metering + integration layer;
native sale_subscription / account_accountant / payment_stripe do all the
financial work (invoicing, HST, dunning, portal, credit notes, Stripe).

Includes:
- Design spec (docs/superpowers/specs/2026-05-27-nexa-billing-centralized-design.md):
  6 locked decisions, architecture, data model, usage engine, Lago-shaped
  API, webhook control loop, NexaCloud pilot, phased dual-run migration.
- Module scaffold: 7 fusion.billing.* models (service, account.link, metric,
  charge, usage, webhook, reconciliation), bearer-auth API controller shell,
  security ACLs, README. Compiles on Odoo 19.0; engine/API bodies are stubs
  pending the implementation plan.
- CLAUDE.md rule #15: no sale.subscription model in Odoo 19 — a subscription
  is a sale.order(is_subscription) + sale.subscription.plan (verified live).

Task 0 verified: a single Stripe account is shared across NexaCloud and all
Lago providers, so no Stripe account/card migration is required.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_centralize_billing/models/reconciliation.py

@@ -0,0 +1,38 @@
+# -*- coding: utf-8 -*-
+# Copyright 2026 Nexa Systems Inc.
+# License OPL-1
+from odoo import fields, models
+
+
+class FusionBillingReconciliation(models.Model):
+    """Dual-run shadow-mode comparison: Odoo-computed vs the app's actual billing.
+
+    During phased cutover (NexaCloud first), Odoo computes invoices while the app
+    keeps charging. This row records the per-customer, per-period delta so we only
+    flip once deltas are within tolerance. See spec §10.
+    """
+
+    _name = "fusion.billing.reconciliation"
+    _description = "Fusion Billing — Dual-Run Reconciliation"
+    _order = "period desc, service_id"
+
+    service_id = fields.Many2one(
+        "fusion.billing.service", required=True, ondelete="cascade", index=True,
+    )
+    partner_id = fields.Many2one("res.partner", required=True, ondelete="cascade", index=True)
+    period = fields.Char(required=True, help="Billing period label, e.g. 2026-05.")
+    odoo_amount = fields.Monetary()
+    external_amount = fields.Monetary(string="App-actual Amount")
+    delta = fields.Monetary(help="odoo_amount - external_amount.")
+    currency_id = fields.Many2one(
+        "res.currency", default=lambda self: self.env.company.currency_id,
+    )
+    status = fields.Selection(
+        [
+            ("match", "Within tolerance"),
+            ("delta", "Delta — investigate"),
+            ("resolved", "Resolved"),
+        ],
+        default="delta", required=True, index=True,
+    )
+    note = fields.Text()