Rename OIDC strategy from :rauthy to :oidc, update callback path

- Rename AshAuthentication strategy from :oidc :rauthy to :oidc :oidc;
  generated actions are now register_with_oidc / sign_in_with_oidc.
- Update config keys (:rauthy → :oidc) in dev.exs and runtime.exs.
- Update default_redirect_uri to /auth/user/oidc/callback everywhere.
- Rename Mv.Accounts helper functions accordingly.
- Update Mv.Secrets, AuthController, link_oidc_account_live and all tests.
- Update docker-compose.prod.yml, .env.example, README and docs.

IMPORTANT: OIDC providers must be updated to use the new redirect URI
/auth/user/oidc/callback instead of /auth/user/rauthy/callback.

.env.example

@@ -22,8 +22,8 @@ ASSOCIATION_NAME="Sportsclub XYZ"
 # These have defaults in docker-compose.prod.yml, only override if needed
 # OIDC_CLIENT_ID=mv
 # OIDC_BASE_URL=http://localhost:8080/auth/v1
-# OIDC_REDIRECT_URI=http://localhost:4001/auth/user/rauthy/callback
-# OIDC_CLIENT_SECRET=your-rauthy-client-secret
+# OIDC_REDIRECT_URI=http://localhost:4001/auth/user/oidc/callback
+# OIDC_CLIENT_SECRET=your-oidc-client-secret
 
 # Optional: OIDC group → Admin role sync (e.g. Authentik groups from profile scope)
 # If OIDC_ADMIN_GROUP_NAME is set, users in that group get Admin role on registration/sign-in.