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/charge.py

@@ -0,0 +1,53 @@
+# -*- coding: utf-8 -*-
+# Copyright 2026 Nexa Systems Inc.
+# License OPL-1
+from odoo import fields, models
+
+
+class FusionBillingCharge(models.Model):
+    """Maps a plan + metric to quota + overage pricing.
+
+    This is where "5,000,000 included / $0.10 per 1k overage" (NexaMaps) or a
+    NexaCloud CPU-seconds quota lives. Keyed by the shared ``plan_code`` the app
+    references; Odoo owns the money, the app owns feature entitlements. See spec §5.1.
+    """
+
+    _name = "fusion.billing.charge"
+    _description = "Fusion Billing — Metered Charge (quota + overage)"
+    _order = "plan_code, name"
+
+    name = fields.Char(required=True)
+    plan_code = fields.Char(
+        required=True, index=True,
+        help="Shared plan_code the source app references (matches a sale.subscription.plan).",
+    )
+    plan_id = fields.Many2one(
+        "sale.subscription.plan",
+        help="Optional link to the Odoo recurrence/plan for this charge.",
+    )
+    metric_id = fields.Many2one(
+        "fusion.billing.metric", required=True, ondelete="restrict",
+    )
+    product_id = fields.Many2one(
+        "product.product", help="Usage product invoiced for overage.",
+    )
+    included_quota = fields.Float(
+        default=0.0, help="Units included before overage applies, per period.",
+    )
+    price_per_unit = fields.Monetary(help="Overage price per unit_batch.")
+    unit_batch = fields.Float(
+        default=1.0, help="Batch size for overage pricing, e.g. 1000 = priced per 1k.",
+    )
+    charge_model = fields.Selection(
+        [
+            ("standard", "Standard (per unit)"),
+            ("graduated", "Graduated"),
+            ("package", "Package"),
+            ("volume", "Volume"),
+        ],
+        default="standard", required=True,
+    )
+    currency_id = fields.Many2one(
+        "res.currency", default=lambda self: self.env.company.currency_id,
+    )
+    active = fields.Boolean(default=True)