Add the flow API: typed messages, git-backed store, REST and live events

Makes the flow engine reachable from the API, which is what M3 needs before
any of it can reach the browser.

- app/flow is a package now; the prototype's watch-dir scripts and the
  matplotlib/networkx visualiser are gone with their dependencies.
- Messages carry a serializable dtype instead of a live Python type, and a
  port name, so the graph can speak qualified names while node functions keep
  local arguments. Redis state is JSON, not pickle.
- Message names are namespaced per flow ("heating.temp"); a bare name resolves
  to its own flow, a dotted one crosses flows.
- Several nodes may provide the same message: producers are a list, so fan-in
  is a real edge instead of a silently dropped one.
- Flows are stored as flow.json plus node sources in a git repository, one
  commit per save, with identical saves skipped so autosave stays quiet.
- Node failures are isolated and reported per node; validate() returns cycles
  and unconnected inputs instead of raising deep in a run.
- Credentials live in an encrypted store and are referenced as {"$secret": …}.
- Engine events reach websocket clients through a bus, so values, node status
  and execution show up live.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016WzrvW7rjQbynnhF6pxh6i

.gitignore

@@ -13,8 +13,8 @@ secrets/
 __pycache__/
 *.py[cod]
 
-# Rendered pipeline graphs (matplotlib output from backend/app/flow).
-pipeline.png
+# The local flow store (its own git repository) and the encrypted secrets file.
+/backend/flow-data/
 
 # Output of `make verify` (root Makefile).
 /frontend/screenshots/