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>

CLAUDE.md

@@ -16,6 +16,8 @@
 7. **Search views**: NO `group expand="0"` syntax.
 8. **SCSS imports**: `@import "./partial"` is FORBIDDEN in Odoo 19 custom SCSS. It prints a warning and silently falls back to the old cached bundle. Register every SCSS file (including `_partial.scss` tokens) as a separate entry in `web.assets_backend`. Put tokens first; Odoo concatenates bundle files so SCSS variables/mixins from the first file are visible to every later file.
 
+15. **There is NO `sale.subscription` model in Odoo 19** (Enterprise `sale_subscription`). A subscription is a **`sale.order`** with `is_subscription=True`, `plan_id` → **`sale.subscription.plan`** (the recurrence), plus `subscription_state` / `next_invoice_date` / `recurring_monthly`. Any Many2one or relation that targets "a subscription" must point at `sale.order` (filter `domain=[('is_subscription','=',True)]`) — **not** `sale.subscription`, which does not exist and fails at install. The surviving `sale.subscription.*` records are only the plan + wizards/reports (`sale.subscription.plan`, `sale.subscription.report`, `sale.subscription.change.customer.wizard`, `sale.subscription.close.reason.wizard`). Verified on live `nexamain` (odoo-nexa, 19.0): `SELECT model FROM ir_model WHERE model LIKE 'sale.subscription%'`.
+
 ## Card Styling — Copy Odoo's Kanban Pattern
 Don't rely on `var(--bs-border-color)` or `var(--bs-body-bg)` for card surfaces — they drift between themes/addons and often render **invisible**. Odoo's own kanban (`.o_kanban_record`) uses **explicit hex** values:
 ```css