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/webhook.py

@@ -0,0 +1,42 @@
+# -*- coding: utf-8 -*-
+# Copyright 2026 Nexa Systems Inc.
+# License OPL-1
+from odoo import fields, models
+
+
+class FusionBillingWebhook(models.Model):
+    """Outbound webhook queue: lifecycle events delivered to source apps.
+
+    Processed by a cron with exponential backoff + HMAC-SHA256 signing, dead-lettered
+    after N attempts (mirror the proven retry pattern in NexaDesk's
+    lago-payment-retry-job). Apps react: suspend / restore / deprovision. See spec §8.
+
+    TODO(spec §8): cron processor, HMAC signing, backoff schedule.
+    """
+
+    _name = "fusion.billing.webhook"
+    _description = "Fusion Billing — Outbound Webhook Event"
+    _order = "create_date desc"
+
+    service_id = fields.Many2one(
+        "fusion.billing.service", required=True, ondelete="cascade", index=True,
+    )
+    event_type = fields.Char(
+        required=True, index=True,
+        help="invoice.payment_failed / invoice.payment_succeeded / "
+             "subscription.terminated / subscription.reactivated / usage.threshold_reached",
+    )
+    payload = fields.Json()
+    state = fields.Selection(
+        [
+            ("pending", "Pending"),
+            ("sent", "Sent"),
+            ("failed", "Failed"),
+            ("dead", "Dead-lettered"),
+        ],
+        default="pending", required=True, index=True,
+    )
+    attempts = fields.Integer(default=0)
+    next_retry_at = fields.Datetime()
+    signature = fields.Char(help="HMAC-SHA256 of the payload using the service webhook_secret.")
+    last_error = fields.Text()