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>

docker-compose.yml

@@ -47,6 +47,10 @@ services:
       PIREON_WEBHOOK_SECRET: ${PIREON_WEBHOOK_SECRET:-}
       PIREON_CLIENT_ID: ${PIREON_CLIENT_ID:-}
       PIREON_CLIENT_SECRET: ${PIREON_CLIENT_SECRET:-}
+      # Pireon clients allow one grant type: the one above is authorization_code,
+      # this one password (email + password sign-in). Optional.
+      PIREON_PASSWORD_CLIENT_ID: ${PIREON_PASSWORD_CLIENT_ID:-}
+      PIREON_PASSWORD_CLIENT_SECRET: ${PIREON_PASSWORD_CLIENT_SECRET:-}
       GRAPHQL_PLAYGROUND: 'true'
       # The SDL / OpenAPI files are only useful in development (codegen); the
       # document is still served at /api/docs-json.