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/web/identity/presenters/src/lib/pages/login.page.ts

@@ -163,7 +163,7 @@ const SAMPLE_FEED = [
             />
           }
 
-          @if (auth.config(); as config) {
+          @if (auth.config()?.clientId) {
             <pi-action-button
               size="w100"
               color="primary"
@@ -172,7 +172,7 @@ const SAMPLE_FEED = [
               [disabled]="busy()"
               (execute)="auth.signInWithPireon(returnUrl())"
             />
-            @if (config.passwordGrant) {
+            @if (passwordEnabled()) {
               <pi-divider color="grey" label="o" gap="m" />
             }
           }