Keep flow state across a restart, and tighten the flow typing

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.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KkmeRiyeYmVZqJVwuyHq9o

docker/compose.dev.yml

@@ -45,6 +45,9 @@ services:
     ports:
       - "127.0.0.1:8080:8080"
 
+  redis:
+    restart: "no"
+
   backend:
     restart: "no"
     ports: