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.

docker-compose.prod.yml

@@ -18,11 +18,11 @@ services:
       PHX_HOST: "${PHX_HOST:-localhost}"
       PORT: "4001"
       PHX_SERVER: "true"
-      # Rauthy OIDC config - use host.docker.internal to reach host services
+      # OIDC config - use host.docker.internal to reach host services
       OIDC_CLIENT_ID: "mv"
       OIDC_BASE_URL: "http://host.docker.internal:8080/auth/v1"
       OIDC_CLIENT_SECRET_FILE: "/run/secrets/oidc_client_secret"
-      OIDC_REDIRECT_URI: "http://localhost:4001/auth/user/rauthy/callback"
+      OIDC_REDIRECT_URI: "http://localhost:4001/auth/user/oidc/callback"
     secrets:
       - db_password
       - secret_key_base