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/presenters/src/lib/http/auth.controller.ts

@@ -30,8 +30,13 @@ import { IssueTokenDto } from './dto/issue-token.dto';
 export class AuthorizationConfigDto {
   @ApiProperty({ example: 'https://oauth.pireon.dev/oauth/authorize' })
   authorizeUrl!: string;
-  @ApiProperty({ description: 'Public id of the task-stream OAuth client' })
-  clientId!: string;
+  @ApiProperty({
+    type: String,
+    nullable: true,
+    description:
+      'Public id of the authorization_code client ("Sign in with Pireon"); null when only email + password is configured',
+  })
+  clientId!: string | null;
   @ApiProperty({ type: [String], example: ['users:read'] })
   scopes!: string[];
   @ApiProperty({ description: 'Whether email + password sign-in is allowed' })