fix(auth): separate Pireon clients for authorization code and password

Pireon OAuth clients allow a single grant type. With one PIREON_CLIENT_ID of
type password, email + password worked but "Sign in with Pireon" failed with
`unauthorized_client`.

- PIREON_CLIENT_ID/SECRET: authorization_code client (Sign in with Pireon,
  Swagger UI). New PIREON_PASSWORD_CLIENT_ID/SECRET: password client, falling
  back to the former so existing setups keep working.
- Refresh tries both clients: a refresh token only works with its issuer.
- PIREON_PASSWORD_GRANT=false now also rejects password grants in the API
  (before it only hid the form).
- /api/auth/config returns clientId: null when there is no authorization_code
  client, and the web hides "Sign in with Pireon".
- docker-compose passes the new variables; docs 05/07, .env.example,
  openapi.json; unit tests for the client selection.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/identity/domain/src/lib/token.ts

@@ -17,11 +17,15 @@ export interface TokenSet {
   tokenType: string;
 }
 
-/** What a browser needs to start the Authorization Code + PKCE flow. */
+/** How a browser can sign in. */
 export interface AuthorizationConfig {
   authorizeUrl: string;
-  clientId: string;
+  /**
+   * Client for Authorization Code + PKCE ("Sign in with Pireon"). Null when
+   * only email + password is configured: Pireon clients allow a single grant.
+   */
+  clientId: string | null;
   scopes: string[];
-  /** Password grant can be disabled on the identity server. */
+  /** Whether email + password sign-in is available. */
   passwordGrant: boolean;
 }