Milestone 2: module/model docs (MD, Sphinx RST, PDF) and work log

74 source modules documented with extracted signatures, JSDoc params,
imports, and call graphs. 10 first-class models (PriceRule through
StatusResponse) have their own MD/RST/PDF. Sphinx HTML builds from
docs/sphinx. Per-module PDFs in docs/modules-pdf and docs/models-pdf.

Middleware gates 2–6 and 10 pass on MOCK_VERAE without NATS.

Learned: RST includes are relative to the RST file; keep one PDF per
module; Add Numbers remains the only push required tomorrow.

docs/sphinx/modules/zappier/invoicing.md

@@ -0,0 +1,75 @@
+# `zappier/invoicing`
+
+**Package:** `zappier`  
+**Source:** `packages/zappier/src/invoicing.ts`  
+**Lines:** 107
+
+## What this module is
+
+Implementation module in `zappier`. The tables below are extracted from the source (signatures + JSDoc).
+
+## Exports
+
+`InvoiceLine`, `Invoice`, `InvoiceRepo`, `InMemoryInvoiceRepo`, `buildInvoice`
+
+## Types / interfaces / classes
+
+| Kind | Name |
+|------|------|
+| interface | `InvoiceLine` |
+| interface | `Invoice` |
+| interface | `InvoiceRepo` |
+| class | `InMemoryInvoiceRepo` |
+
+## Functions
+
+| Name | Parameters | Param types (JSDoc) | Returns | Calls (same file / helpers) |
+|------|------------|---------------------|---------|-----------------------------|
+| `buildInvoice` | `args: {
+  customer: Customer;
+  period: string;
+  sequence: number;
+  entries: UsageEntry[];
+  tier: TierConfig;
+  poNumber?: string;
+}` | — | `unknown` | see Call graph |
+
+## Methods (class / object)
+
+| Name | Parameters |
+|------|------------|
+| `save` | `invoice: Invoice` |
+| `get` | `id: string` |
+| `list` | `filter: {
+    customerId?: string;
+    period?: string;
+    status?: Invoice['status'];
+  }` |
+| `nextSequence` | `period: string` |
+| `save` | `invoice: Invoice` |
+| `get` | `id: string` |
+| `list` | `filter: {
+    customerId?: string;
+    period?: string;
+    status?: Invoice['status'];
+  }` |
+| `nextSequence` | `period: string` |
+
+## What it imports / requires
+
+- `./auth`
+- `./pricing`
+- `./usage`
+
+## Call graph (identifiers invoked)
+
+`cents`, `save`, `get`, `list`, `nextSequence`, `set`, `map`, `values`, `filter`, `sort`, `localeCompare`, `buildInvoice`, `reduce`, `min`, `padStart`
+
+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`.
+