The engine fell back to MemoryState because there was no Redis to talk
to, so every value was lost on restart. Adds a redis service with
append-only persistence, reachable only from the backend, and points
REDIS_HOST at it.
state.py and pipeline.py leave the strict-mypy exclusion. redis-py types
its calls as sync/async unions; the casts narrow that at the points of
use rather than switching to redis.asyncio, which would make a
deliberately synchronous backend asynchronous. nodes.py stays excluded.
The .env.example CORS entry belongs with the CI work that follows.
Melvin Strobl
52fe9c4c0b