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>

apps/api/openapi.json

@@ -3879,7 +3879,8 @@
           },
           "clientId": {
             "type": "string",
-            "description": "Public id of the task-stream OAuth client"
+            "nullable": true,
+            "description": "Public id of the authorization_code client (\"Sign in with Pireon\"); null when only email + password is configured"
           },
           "scopes": {
             "example": [