docs(erp-toolkit): expand use cases with the Wilken integration set

[anttiviljami]

Summary

Expands docs/integrations/erp-toolkit/use-cases.md to cover the full set of inbound and outbound use cases that production integrations (audited against the live Wilken integration via the Integration Toolkit API) actually ship today.

Inbound additions / clarifications

• Sync Installment Schedule — new section. Documents the Abschlagsplan flow that mirrors the upcoming installment plan into billing_event rows with direction: "debit" and stable DEDUCTION-<accountId>-<date> external IDs.
• Sync Portal User — new section. Documents the registration/SSO event that provisions a PROFILE contact + portal_user and links the portal user to the business-partner contact via mapped_contact.
• Sync Billing Events — enriched. Now covers the billing_account.balance update and the direction (debit / credit) attribute, and calls out that deterministic external_ids drive deduplication.
• Keep Billing Account In Sync — clarified. Calls out the common pattern of splitting this into three sub-domain mappings (account header / billing address / payment method) and documents the masked-IBAN + _tags: [DEBIT, CREDIT] convention.
• Sync Documents — clarified. Notes that File Proxy via custom_download_url is the standard approach when not migrating the binary content.

Outbound additions / clarifications

• Submit Service Meter Reading — new section (ServiceMeterReadingAdded). The ticket-driven service-agent variant of meter reading submission.
• Submit New Order — new section (OrderSubmission). Journey-driven acquisition flow, documents the supplyConnectionData.reason mapping (CHANGE_OF_SUPPLIER, MOVE_IN, RATE_CHANGE).
• Switch Tariff — new section (TariffChange). Tariff/product switch on an existing contract.
• Remove Billing Account Connection — new section (BillingAccountConnectionRemoved). Portal-side unlink of a billing account from the portal user.
• Submit General Request — new section (GeneralRequestCreated). Contact-form ticket forwarded to the ERP, routed via requestTypeKey.
• Request Data Sync — replaces the previous "custom webhook" placeholder with the real OnDemandSyncCustomerRequested and OnDemandSyncContractRequested events that already exist in the core event catalog.
• Existing outbound sections — payload-field lists realigned with the actual context block (identityId, customerAccountId, contractNumber) and data payloads observed in production.
• Payment Method change — new tip noting the webhook carries context only (no IBAN), and that the middle layer should re-fetch the billing account from epilot to read the new payment method.

Quick Reference tables

Both inbound and outbound tables in Quick Reference are extended to list every new use case with its core entities, core event, and typical trigger.

How this was verified

• Audited the live Wilken integration via epilot integration-toolkit listIntegrationsV2 (22 use cases — 10 inbound, 12 outbound) and reviewed each JSONata mapping for fields, identifiers and entity relations.
• Confirmed every new event referenced (ServiceMeterReadingAdded, OrderSubmission, TariffChange, BillingAccountConnectionRemoved, GeneralRequestCreated, OnDemandSyncCustomerRequested, OnDemandSyncContractRequested) is already documented in docs/integrations/core-events.md. No new events needed to be added.
• Confirmed <EventSchemaViewer> and <EntitySchemaViewer> render <div id={…}> so all #anchor links resolve.
• Ran npm run build — clean Docusaurus build, no broken-link or broken-anchor warnings.

Test plan

• npm run build succeeds with no broken-link warnings
• Reviewer spot-checks 1-2 new sections against the live Wilken Integration Hub UI
• Reviewer confirms naming ("Sync Portal User" rather than "identity") matches the entity wording the team prefers