Docs: Vereinfacht API integration and guidelines

- CODE_GUIDELINES: add vereinfacht/ to project structure, required-fields note, link to vereinfacht-api
- docs/vereinfacht-api.md: filter API, minimal create payload, no extra required fields
- feature-roadmap: member-contact sync implemented, link to doc

CODE_GUIDELINES.md

@@ -62,6 +62,7 @@ We are building a membership management system (Mila) using the following techno
 
 **Related documents:**
 - **UI / UX:** [`DESIGN_DUIDELINES.md`](../DESIGN_DUIDELINES.md) defines visual and interaction consistency: use of CoreComponents (no raw DaisyUI in views), page skeleton (`<.header>`, `mt-6 space-y-6`), **Back button left in header for edit/new forms** (§2.2), typography, buttons, forms, tables, flash/toast, and microcopy (e.g. German "du" and glossary). Follow "components first" and semantic variants instead of hard-coded colors.
+- **Vereinfacht API:** [`docs/vereinfacht-api.md`](docs/vereinfacht-api.md) describes the finance-contact sync (find by email filter, minimal create payload, no extra required member fields).
 
 ---
 
@@ -115,8 +116,13 @@ lib/
 │   ├── membership_fees/   # Membership fee business logic
 │   │   ├── cycle_generator.ex  # Cycle generation algorithm
 │   │   └── calendar_cycles.ex  # Calendar cycle calculations
+│   ├── vereinfacht/       # Vereinfacht accounting API integration
+│   │   ├── client.ex      # HTTP client (finance-contacts: create, update, find by email)
+│   │   ├── vereinfacht.ex # Business logic (sync_member, sync_members_without_contact)
+│   │   ├── sync_flash.ex  # Flash message helpers for sync results
+│   │   └── changes/        # Ash changes (SyncContact, sync linked member)
 │   ├── helpers.ex         # Shared helper functions (ash_actor_opts)
-│   ├── constants.ex       # Application constants (member_fields, custom_field_prefix)
+│   ├── constants.ex       # Application constants (member_fields, custom_field_prefix, vereinfacht_required_member_fields)
 │   ├── application.ex     # OTP application
 │   ├── mailer.ex          # Email mailer
 │   ├── release.ex         # Release tasks
@@ -2874,7 +2880,7 @@ Building accessible applications ensures that all users, including those with di
 
 **Required Fields:**
 
-Which member fields are required (asterisk, tooltip, validation) is configured in **Settings** (Memberdata section: edit a member field and set "Required"). The member create/edit form and Member resource validation both read `settings.member_field_required`. Email is always required; other fields default to optional.
+Which member fields are required (asterisk, tooltip, validation) is configured in **Settings** (Memberdata section: edit a member field and set "Required"). The member create/edit form and Member resource validation both read `settings.member_field_required`. Email is always required; other fields default to optional. The Vereinfacht integration does not add extra required member fields (the external API accepts a minimal payload when creating contacts and supports filter-by-email for lookup).
 
 ```heex
 <!-- Mark required fields (value from settings or always true for email) -->

docs/feature-roadmap.md

@@ -247,7 +247,7 @@
 - ❌ Payment records/transactions (external payment tracking)
 - ❌ Payment reminders
 - ❌ Invoice generation
-- ❌ vereinfacht.digital API integration
+- ✅ Member–finance-contact sync with vereinfacht.digital API (see `docs/vereinfacht-api.md`); ❌ transaction import / full API integration
 - ❌ SEPA direct debit support
 - ❌ Payment reports
 

docs/vereinfacht-api.md

@@ -0,0 +1,47 @@
+# Vereinfacht API Integration
+
+This document describes the current integration with the Vereinfacht (verein.visuel.dev) accounting API for syncing members as finance contacts.
+
+## Overview
+
+- **Purpose:** Create and update external finance contacts in Vereinfacht when members are created or updated; support bulk sync for members without a contact ID.
+- **Configuration:** ENV or Settings: `VEREINFACHT_API_URL`, `VEREINFACHT_API_KEY`, `VEREINFACHT_CLUB_ID`, optional `VEREINFACHT_APP_URL` for contact view links.
+- **Modules:** `Mv.Vereinfacht` (business logic), `Mv.Vereinfacht.Client` (HTTP client), `Mv.Vereinfacht.Changes.SyncContact` (Ash after_transaction change).
+
+## API Usage
+
+### Finding an existing contact by email
+
+The API supports filtered list requests. Use a single GET instead of paginating:
+
+- **Endpoint:** `GET /api/v1/finance-contacts?filter[isExternal]=true&filter[email]=<email>`
+- **Client:** `Mv.Vereinfacht.Client.find_contact_by_email/1` builds this URL (with encoded email) and returns `{:ok, contact_id}` if the first match exists, `{:error, :not_found}` otherwise.
+- No member fields are required in the app solely for this lookup.
+
+### Creating a contact
+
+When creating an external finance contact, the API only requires:
+
+- **Attributes:** `contactType` (e.g. `"person"`), `isExternal: true`
+- **Relationship:** `club` (club ID from config)
+
+Additional attributes (firstName, lastName, email, address, zipCode, city) are optional and are sent when present on the member so the contact is filled in. The app does **not** enforce extra required member fields for Vereinfacht; only Settings-based required fields and email apply.
+
+- **Client:** `Mv.Vereinfacht.Client.create_contact/1` builds the JSON:API body from the member; `Mv.Constants.vereinfacht_required_member_fields/0` is an empty list.
+
+### Updating a contact
+
+- **Endpoint:** `PATCH /api/v1/finance-contacts/:id`
+- **Client:** `Mv.Vereinfacht.Client.update_contact/2` sends current member attributes. The API may still validate presence/format of fields on update.
+
+## Flow
+
+1. **Member create/update:** `SyncContact` runs after the transaction. If the member has no `vereinfacht_contact_id`, the client tries `find_contact_by_email(email)`; if found, it updates that contact and stores the ID on the member; otherwise it creates a contact and stores the new ID. If the member already has a contact ID, the client updates the contact.
+2. **Bulk sync:** “Sync all members without Vereinfacht contact” calls `Vereinfacht.sync_members_without_contact/0`, which loads members with nil/blank `vereinfacht_contact_id` and runs the same create/update flow per member.
+
+## References
+
+- **Config:** `Mv.Config` (`vereinfacht_api_url`, `vereinfacht_api_key`, `vereinfacht_club_id`, `vereinfacht_app_url`, `vereinfacht_configured?/0`).
+- **Constants:** `Mv.Constants.vereinfacht_required_member_fields/0` (empty), `vereinfacht_required_field?/1`.
+- **Tests:** `test/mv/vereinfacht/`, `test/mv/config_vereinfacht_test.exs`; see `test/mv/vereinfacht/vereinfacht_test_README.md` for scope.
+- **Roadmap:** Payment/transaction import and deeper integration are tracked in `docs/feature-roadmap.md` and `docs/membership-fee-architecture.md`.