CUST-WP-0058: T04 approved, T05 client-hub scoping, T06 fin-hub gap done

Service contract v0.1 accepted (Bernd 2026-07-10). Client-hub scoped as
core-hub extension (client/engagement/app_instance/support_state, CLNT-WP
skeleton). fin-hub gap assessment: 5 gaps, FIN-WP-0002 skeleton posted to
fin-hub lane (msg 9c8e8ed4); recommendation fin-hub stays reporting-only.
Remaining: T07 delivery-lane pilot, T08 closeout.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/fin-hub-client-billing-gap.md

@@ -0,0 +1,53 @@
+# fin-hub Client-Billing Gap Assessment
+
+**Created:** 2026-07-10
+**Workplan:** `CUST-WP-0058-T06`
+**Baseline reviewed:** `FIN-WP-0000/0001` (finished) — models `budget.py`,
+`service_cost.py`; HostEurope CSV ingest; runway calculator; fin→ops coupling.
+**Contract obligations:** business-app service contract §5 (cost attribution
+key `client × app × instance`, per-instance cost reporting).
+
+## What exists
+
+- `ServiceCost`: per-service infra cost, keyed `service_id × environment ×
+  period_month` (EUR, source-tagged) — fin→ops attribution works.
+- Budgets, commitments, runway projection with alert thresholds.
+- Manual CSV ingest (HostEurope) as designed for v0.1.
+
+## Gaps vs. commercial operation
+
+| # | Gap | Detail | Severity |
+| --- | --- | --- | --- |
+| G1 | **No client dimension** | `ServiceCost` cannot express `client × app × instance` (§5.1). Instance-per-client (DR-1 C) makes this the primary reporting axis. | high |
+| G2 | **No revenue side** | Costs and budgets only; engagement pricing is not recorded, so margin-per-client — the input to DR-1's cost-optimization decision — cannot be computed. | high |
+| G3 | **No per-instance cost split** | HostEurope CSV gives host-level cost; client instances share cluster nodes. An allocation method (namespace/resource-based split) is needed for §5.2. | medium |
+| G4 | **Invoicing boundary undecided** | fin-hub SCOPE excludes payment/invoicing. Something must own invoice generation for clients. | medium |
+| G5 | **No client-hub reference seam** | Once client-hub exists, cost rows should reference its `app_instance` / cost key instead of free-form strings. | low (sequencing) |
+
+## Boundary recommendation (G4)
+
+Keep fin-hub **reporting-only**: it owns attribution, margin, and runway; an
+external bookkeeping/invoicing system (e.g. lexoffice/sevDesk — Bernd's
+choice, GoBD-relevant) owns invoice generation and payment tracking. fin-hub
+provides the per-engagement billing basis (attributed cost + entitlement
+price) as an export, never executes payments. This keeps constitution §4
+financial custody with the human/GmbH.
+
+## Proposed workplan skeleton (for `/home/worsch/fin-hub`)
+
+```
+FIN-WP-0002 — client attribution and billing basis
+  T01  Add client attribution: cost rows carry client/app/instance key
+       (string key now; client-hub FK seam later, G5) (todo, high)
+  T02  Engagement revenue model: price per engagement period, margin view
+       per client × app (todo, high)
+  T03  Per-instance allocation method for shared infra (namespace-based
+       split; documented assumptions) (todo, medium)
+  T04  Billing-basis export per client × period (feeds external invoicing;
+       no payment execution) (todo, medium)
+  T05  Decision + doc: external invoicing system selection (human gate,
+       GoBD) (wait, high)
+```
+
+Posted to the fin-hub lane via State Hub message; implementation belongs in
+the fin-hub repo (ADR-001 — the workplan file originates there).