adr(webfinger): Document OIDC client parameter discovery

docs/adr/0003-oidc-client-config-discovery.md

@@ -0,0 +1,67 @@
+---
+title: "Discover OIDC Client configuration via WebFinger"
+---
+
+* Status: pending
+* Deciders: [@TheOneRing @kulmann @rhafer @dragotin]
+* Date: 2026-02-02
+
+Reference: https://github.com/opencloud-eu/opencloud/pull/2072, https://github.com/opencloud-eu/desktop/issues/217
+
+## Context and Problem Statement
+
+Up to now our client applications used hard-coded OIDC client configurations.
+So it is not possible to change the client id that a client should use or the
+list of scopes that a client needs to request. This makes it hard to integrate
+OpenCloud with various existing identity providers. For example:
+
+- Authentik basically creates a different issuer URL for each client. As OpenCloud
+  can only work with a single issuer URL, all OpenCloud clients need to use the
+  same client id to work with Authetnik.
+- Some IDPs (kanidm) are not able to work with user-supplied client ids. They generate
+  client ids automatically and do not allow to specify them manually.
+- To make features like automatic role assignment work, clients need to request
+  specific scopes, depending on which exact IDP is used.
+
+## Decision Drivers
+
+* Support broader set of IDPs
+* Do required the user got configure anything additional on the client side
+
+## Decision
+
+Enhance the WebFinger service in OpenCloud to provide platform-specific OIDC
+discovery, enabling clients to query for the correct OIDC `client_id` and
+`scopes` based on their application type (e.g., web, desktop, android, ios).
+
+This is achieved by allowing and additional `platform` query parameter to be used
+when querying the WebFinger endpoint. The response will include the appropriate
+`client_id` and `scopes` in the `properties` section of the response.
+
+This is implemented in a backward-compatible way, so existing clients that do not
+specify the `platform` parameter will continue to receive just the issuer information.
+
+## Example
+
+### Client Request
+
+```
+GET /.well-known/webfinger?resource=https://cloud.opencloud.test&rel=http://openid.net/specs/connect/1.0/issuer&platform=desktop
+```
+
+### Example Response
+
+```json
+{
+  "subject": "https://cloud.opencloud.test",
+  "links": [{
+    "rel": "http://openid.net/specs/connect/1.0/issuer",
+    "href": "https://idp.example.com"
+  }],
+  "properties": {
+    "http://opencloud.eu/ns/oidc/client_id": "desktop-client-id",
+    "http://opencloud.eu/ns/oidc/scopes": ["openid", "profile", "email", "offline_access"]
+  }
+}
+```
+