docs: name the app-side service token SEAMLESS_SERVICE_TOKEN

[Bccorb]

Summary

The app-side service token env var is now SEAMLESS_SERVICE_TOKEN. The auth API server still reads API_SERVICE_TOKEN; both hold the same value.

• README quickstart and options table read the service token from SEAMLESS_SERVICE_TOKEN (was SERVICE_SECRET / API_SERVICE_TOKEN), with one sentence explaining the two names.
• The FastAPI and Django README examples and the Django module docstring read SEAMLESS_SERVICE_TOKEN.
• The conformance reference apps (conformance/_shared.py) and their CI step read SEAMLESS_SERVICE_TOKEN. The conformance stack's auth API container is configured by seamless-cli and still uses API_SERVICE_TOKEN with the same value.

The adapter reads no environment variables itself; these are app-side example and fixture names only.

Checks

• uv run ruff check ., uv run ruff format --check ., uv run mypy, uv run pytest pass.
• Conformance was not run locally (ports 5312 and 8080 are in use here); the CI conformance job covers the env rename.