Add NATS account-balance SoT and statement review for customers, CS, and sales

Internal billing now uses verae.billing.* request-reply and pubs. zappier-account-balance tracks prepaid, credits, usage, and payments. Portal, admin, CS, and sales all review the same statement. Independent Forgejo repos stay split via push-module-repos.

docs/sphinx/modules/zappier/billing-nats.md

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+# `zappier/billing-nats`
+
+**Package:** `zappier`  
+**Source:** `packages/zappier/src/billing-nats.ts`  
+**Lines:** 74
+
+## What this module is
+
+Implementation module in `zappier`. The tables below are extracted from the source (signatures + JSDoc).
+
+## Exports
+
+`BILLING_SUBJECTS`, `BillingStatement`, `natsStatement`, `natsAdjust`, `natsPublish`
+
+## Types / interfaces / classes
+
+| Kind | Name |
+|------|------|
+| type | `BillingStatement` |
+| type | `Nc` |
+
+## Functions
+
+| Name | Parameters | Param types (JSDoc) | Returns | Calls (same file / helpers) |
+|------|------------|---------------------|---------|-----------------------------|
+| `nc` | `(none)` | — | `unknown` | see Call graph |
+| `encode` | `obj: unknown` | — | `unknown` | see Call graph |
+| `decode` | `buf: Uint8Array` | — | `unknown` | see Call graph |
+| `natsStatement` | `customerId: string` | — | `unknown` | see Call graph |
+| `natsAdjust` | `payload: {
+  customerId: string;
+  cents: number;
+  reason: string;
+  agent: string;
+  kind?: string;
+}` | — | `unknown` | see Call graph |
+| `natsPublish` | `subject: string, payload: unknown` | — | `unknown` | see Call graph |
+
+## What it imports / requires
+
+_No imports detected._
+
+## Call graph (identifiers invoked)
+
+`unset`, `nc`, `async`, `import`, `connect`, `split`, `encode`, `stringify`, `decode`, `parse`, `natsStatement`, `request`, `natsAdjust`, `natsPublish`, `then`, `publish`
+
+Each identifier is a call site in this file. Follow the import list to see the defining module; open that module’s MD for parameter and return types.
+
+## Return values (how to read this)
+
+- HTTP route handlers return Express `res.json(...)` bodies (see route docs).
+- Zapier `perform` functions return a **single object** (creates) or an **array** (triggers/searches).
+- Pricing functions return integer **cents** on `Quote.totalCents`.
+