Record a short value history per message

The panel is going to sparkline how a value moved, and nothing kept more
than the latest one. Emissions now append to a capped list beside the
value — 120 points, enough to fill a sparkline without making Redis a
time-series store — and `/flows/{flow}/history/{message}` hands it back
oldest first.

Only numbers are recorded, bools included as neither, so a string
message costs nothing at all. The response carries `numeric` so an empty
series reads as "not plottable" rather than "nothing yet". The append is
one pipelined round-trip per emission batch and sits outside the lock,
since the list is append-only.

MemoryState keeps the same window in a deque, so the endpoint answers
without Redis too.

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

backend/app/api/routes/flows.py

@@ -26,11 +26,14 @@ from app.flow.schemas import (
     FlowsPublic,
     FlowStatePublic,
     FlowSummary,
+    HistoryPoint,
+    MessageHistory,
     MessageValue,
     NodeSource,
     NodeStatusPublic,
     NodeTypeInfo,
 )
+from app.flow.state import as_number
 from app.flow.store import FlowExists, FlowNotFound
 from app.models import Message
 
@@ -275,6 +278,29 @@ def read_flow_state(name: str, controller: FlowControllerDep) -> Any:
     return _flow_state(controller, name)
 
 
+@router.get("/{name}/history/{message}", response_model=MessageHistory)
+def read_message_history(
+    name: str,
+    message: str,
+    controller: FlowControllerDep,
+) -> Any:
+    """The recent values of one message, for plotting.
+
+    ``message`` may be given bare or qualified; a message that never carried a
+    number comes back with an empty series.
+    """
+    _read_flow(controller, name)
+    key = qualify(name, message)
+    return MessageHistory(
+        message=key,
+        numeric=as_number(controller.state.get(key)) is not None,
+        points=[
+            HistoryPoint(ts=ts, value=value)
+            for ts, value in controller.state.history(key)
+        ],
+    )
+
+
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