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

backend/pyproject.toml

@@ -44,11 +44,11 @@ build-backend = "hatchling.build"
 strict = true
 exclude = ["venv", ".venv", "alembic"]
 
-# app/flow/nodes.py still carries the prototype's node classes, and state.py's
-# redis calls type as sync/async unions under the current stubs. Both are being
-# revisited; every module around them is checked strictly.
+# app/flow/nodes.py still carries the prototype's node classes: the integration
+# nodes need annotations of their own, plus stubs for croniter and influxdb_client.
+# It is being revisited per integration; every module around it is checked strictly.
 [[tool.mypy.overrides]]
-module = ["app.flow.nodes", "app.flow.pipeline", "app.flow.state"]
+module = ["app.flow.nodes"]
 ignore_errors = true
 
 [tool.ruff]