feat(authentik): capture MAS OIDC Provider/Application as a Blueprint

Closes issue #36. The OAuth2 Provider ("Matrix Authentication Service")
and its Application ("matrix" slug) linking Authentik to MAS were
originally clicked together by hand in the UI and existed nowhere as
code - unlike the flow fixes already captured in authentik-blueprints.yaml.
Losing the Authentik DB would have meant re-creating this from scratch,
including a fresh client_secret that MAS would no longer match.

The client_secret is read via !Env from AUTHENTIK_MAS_OIDC_CLIENT_SECRET,
sourced from a new key in the existing SOPS-encrypted authentik-credentials
Secret, rather than inlined into the blueprint ConfigMap (which isn't
itself encrypted). Value used is the actual live secret already in use,
read directly from the running Authentik DB - not a new/rotated one, so
this changes nothing about the current MAS<->Authentik pairing.

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

apps/authentik/authentik-blueprints.yaml

@@ -226,3 +226,58 @@ data:
           domain: authentik-default
         attrs:
           default_application: !Find [authentik_core.application, [slug, matrix]]
+  matrix-oidc-provider.yaml: |
+    # yaml-language-server: $schema=https://goauthentik.io/blueprints/schema.json
+    version: 1
+    metadata:
+      name: matrix-oidc-provider
+      labels:
+        blueprints.goauthentik.io/instantiate: "true"
+    entries:
+      # The OIDC Provider + Application linking Authentik to MAS was originally
+      # clicked together by hand in the UI and existed nowhere as code (issue
+      # #36): losing the Authentik DB would have meant re-creating this from
+      # scratch, including a new client_secret that MAS would then no longer
+      # match. client_secret is read from AUTHENTIK_MAS_OIDC_CLIENT_SECRET
+      # (see authentik.yaml HelmRelease values) rather than inlined here,
+      # since this ConfigMap itself is not SOPS-encrypted - the actual value
+      # lives in the authentik-credentials Secret instead.
+      - model: authentik_providers_oauth2.oauth2provider
+        state: present
+        identifiers:
+          name: Matrix Authentication Service
+        id: matrix_mas_provider
+        attrs:
+          client_type: confidential
+          client_id: dHbTAgAgXvjh3VALh220mB3dxcVXAifiXU2ZO3U6
+          client_secret: !Env AUTHENTIK_MAS_OIDC_CLIENT_SECRET
+          # Path includes MAS's own upstream-provider ID, not Authentik's -
+          # must match MAS's config exactly or the OIDC callback breaks.
+          redirect_uris:
+            - matching_mode: strict
+              url: https://account.axion1337.chat/upstream/callback/01KQDJTR1ZVTG8JQ220F5BNBFZ
+          # Stable across username renames - this is what keeps
+          # upstream_oauth_links rows valid after e.g. the elbojoloco rename.
+          sub_mode: hashed_user_id
+          include_claims_in_id_token: true
+          access_code_validity: minutes=1
+          access_token_validity: minutes=5
+          signing_key: !Find [authentik_crypto.certificatekeypair, [name, "authentik Self-signed Certificate"]]
+          authorization_flow: !Find [authentik_flows.flow, [slug, default-provider-authorization-implicit-consent]]
+          invalidation_flow: !Find [authentik_flows.flow, [slug, default-provider-invalidation-flow]]
+          property_mappings:
+            - !Find [authentik_core.propertymapping, [managed, "goauthentik.io/providers/oauth2/scope-openid"]]
+            - !Find [authentik_core.propertymapping, [managed, "goauthentik.io/providers/oauth2/scope-email"]]
+            - !Find [authentik_core.propertymapping, [managed, "goauthentik.io/providers/oauth2/scope-profile"]]
+
+      - model: authentik_core.application
+        state: present
+        identifiers:
+          slug: matrix
+        attrs:
+          name: aXion1337.chat Accountverwaltung
+          provider: !KeyOf matrix_mas_provider
+          meta_description: Matrixclient tailored for aXionCommunity
+          meta_publisher: aXionGaming
+          policy_engine_mode: any
+          open_in_new_tab: false