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/__manifest__.py

@@ -0,0 +1,54 @@
+# -*- coding: utf-8 -*-
+# Copyright 2026 Nexa Systems Inc.
+# License OPL-1 (Odoo Proprietary License v1.0)
+{
+    "name": "Fusion Centralized Billing",
+    "version": "19.0.1.0.0",
+    "category": "Accounting/Subscriptions",
+    "summary": "Centralized billing engine for all NexaSystems services — metered usage, "
+               "per-app billing API, and outbound webhooks on top of Odoo Enterprise subscriptions.",
+    "description": """
+Fusion Centralized Billing
+==========================
+
+Makes this Odoo Enterprise instance the single billing brain for every NexaSystems
+service (NexaCloud hosting, NexaDesk chat, NexaMaps API, custom apps, memberships).
+
+It adds ONLY the metering + integration layer; all financial behaviour (invoicing,
+HST tax, proration, dunning, portal, credit notes, Stripe) is native Odoo Enterprise.
+
+Capabilities
+------------
+* Service registry — one record per source app (NexaCloud / NexaDesk / NexaMaps) with
+  bearer API key + webhook config.
+* Identity links — fold each app's external account into one ``res.partner``.
+* Metric + Charge catalog — billable metrics with quota + overage pricing, keyed by a
+  shared ``plan_code`` (apps own feature entitlements; Odoo owns money).
+* Usage engine — aggregate-push: apps send periodic counters; a pre-invoice cron feeds
+  billable quantities onto the subscription ``sale.order``.
+* Inbound API — Lago-shaped REST (``/api/billing/v1/*``), bearer auth.
+* Outbound webhooks — HMAC-signed lifecycle events (payment failed/succeeded,
+  subscription terminated) so apps suspend / restore / deprovision.
+
+Design spec: docs/superpowers/specs/2026-05-27-nexa-billing-centralized-design.md
+
+Status: SCAFFOLD. Model fields are in place; engine/API/webhook bodies are stubs to be
+implemented via the writing-plans output. Per repo CLAUDE.md, read live Odoo 19
+reference files from the container before implementing subscription/account internals.
+    """,
+    "author": "Nexa Systems Inc.",
+    "website": "https://nexasystems.ca",
+    "license": "OPL-1",
+    "depends": [
+        "account_accountant",
+        "sale_subscription",
+        "sale_management",
+        "payment_stripe",
+    ],
+    "data": [
+        "security/ir.model.access.csv",
+    ],
+    "installable": True,
+    "application": False,
+    "auto_install": False,
+}