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

backend/tests/flow/test_pipeline.py

@@ -0,0 +1,152 @@
+"""The wiring fundamentals: name binding, fan-in, namespaces, validation."""
+
+from app.flow.messages import DType, MessageSpec
+from app.flow.nodes import Node
+from app.flow.pipeline import Pipeline
+
+
+def spec(name: str, dtype: DType = DType.FLOAT, port: str = "") -> MessageSpec:
+    return MessageSpec(name=name, dtype=dtype, port=port)
+
+
+def make_node(node_id: str, flow: str, f, requires=(), provides=()) -> Node:
+    node = Node(f=f, requires=list(requires), provides=list(provides), name=node_id)
+    node.assign_flow(flow, node_id)
+    return node
+
+
+def test_bare_names_are_scoped_to_their_flow():
+    source = make_node(
+        "source", "heating", lambda params: {"temp": 20.0}, provides=[spec("temp")]
+    )
+    assert "heating.temp" in source.provides
+
+
+def test_consumer_receives_from_any_producer():
+    # Two producers of one message: each publication reaches the consumer.
+    seen = []
+
+    def emit_a(params):
+        return {"temp": 1.0}
+
+    def emit_b(params):
+        return {"temp": 2.0}
+
+    def consume(temp, params):
+        seen.append(temp)
+        return None
+
+    a = make_node("a", "heating", emit_a, provides=[spec("temp")])
+    b = make_node("b", "heating", emit_b, provides=[spec("temp")])
+    c = make_node("c", "heating", consume, requires=[spec("temp")])
+
+    pipeline = Pipeline(nodes=[a, b, c])
+
+    assert pipeline.produces["heating.temp"] == [a, b]
+    assert pipeline.dependencies[c] == frozenset({a, b})
+
+    a.inject()
+    b.inject()
+
+    assert seen == [1.0, 2.0]
+    # Latest value wins.
+    assert pipeline.state["heating.temp"] == 2.0
+
+
+def test_flows_connect_through_qualified_names():
+    def emit(params):
+        return {"power": 500.0}
+
+    def consume(power, params):
+        return {"used": power}
+
+    producer = make_node("meter", "solar", emit, provides=[spec("power")])
+    # A bare name would be heating.power; the dotted one crosses the flow.
+    consumer = make_node(
+        "load",
+        "heating",
+        consume,
+        requires=[spec("solar.power")],
+        provides=[spec("used")],
+    )
+
+    pipeline = Pipeline(nodes=[producer, consumer])
+    assert pipeline.dependencies[consumer] == frozenset({producer})
+
+    producer.inject()
+    assert pipeline.state["heating.used"] == 500.0
+
+
+def test_ports_keep_function_arguments_local():
+    def convert(celsius, params):
+        return {"fahrenheit": celsius * 9 / 5 + 32}
+
+    node = make_node(
+        "convert",
+        "heating",
+        convert,
+        requires=[spec("solar.celsius", port="celsius")],
+        provides=[spec("fahrenheit")],
+    )
+    Pipeline(nodes=[node])
+
+    assert node.execute({"solar.celsius": 100.0}) == {"heating.fahrenheit": 212.0}
+
+
+def test_validate_reports_cycles_and_dangling_inputs():
+    a = make_node(
+        "a", "f", lambda b, params: {"a": b}, requires=[spec("b")], provides=[spec("a")]
+    )
+    b = make_node(
+        "b", "f", lambda a, params: {"b": a}, requires=[spec("a")], provides=[spec("b")]
+    )
+    lonely = make_node(
+        "lonely", "f", lambda missing, params: None, requires=[spec("missing")]
+    )
+
+    issues = Pipeline(nodes=[a, b, lonely]).validate()
+    codes = {issue.code for issue in issues}
+
+    assert "cycle" in codes
+    assert "unconnected_input" in codes
+    dangling = next(i for i in issues if i.code == "unconnected_input")
+    assert dangling.message_name == "f.missing"
+    assert dangling.node == "f.lonely"
+
+
+def test_declared_flow_inputs_are_not_dangling():
+    node = make_node(
+        "n", "f", lambda setpoint, params: None, requires=[spec("setpoint")]
+    )
+    issues = Pipeline(nodes=[node]).validate(known_inputs={"f.setpoint"})
+    assert issues == []
+
+
+def test_a_failing_node_does_not_stop_its_siblings():
+    ran = []
+
+    def boom(params):
+        raise RuntimeError("nope")
+
+    def fine(params):
+        ran.append("fine")
+        return {"ok": 1.0}
+
+    bad = make_node("bad", "f", boom, provides=[spec("bad_out")])
+    good = make_node("good", "f", fine, provides=[spec("ok")])
+
+    pipeline = Pipeline(nodes=[bad, good])
+    pipeline.run()
+
+    assert ran == ["fine"]
+    assert pipeline.state["f.ok"] == 1.0
+
+
+def test_values_carry_timestamps():
+    node = make_node("n", "f", lambda params: {"out": 1.0}, provides=[spec("out")])
+    pipeline = Pipeline(nodes=[node])
+    node.inject()
+
+    values = pipeline.values("f")
+    assert values["f.out"]["value"] == 1.0
+    assert values["f.out"]["ts"] > 0