Hold a node down for a minute before calling it down

A connector that misses one poll turned the health badge amber and the node
on the canvas red until it recovered. `LoadedNode.health_since` marks when the
status last actually changed — a rotating error detail under an unchanged
status does not move it, or a connector retrying with a different errno each
poll would never debounce — and `unhealthy_nodes` applies a 60 s floor to it.

The floor is on the surfacing, not on the transition: the stored health is the
truth the moment a node reports it, so a flow reading node health is never
told a stale story. Only going down is held back; recovery clears at once.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CL9zvnnvcp1mvA8o7impxk

backend/fluksio/api/routes/observability.py

@@ -182,7 +182,10 @@ async def read_summary(request: Request, controller: FlowControllerDep) -> Any:
     paused = set(controller.paused_flows())
     entries = list(controller.loaded.values())
     errored = [e for e in entries if e.status is NodeStatus.ERROR]
-    unhealthy = [e for e in entries if e.health == "down"]
+    # Not `e.health == "down"` directly: a node has to have been down for
+    # `HEALTH_FLOOR` before it counts, so a device that misses one poll does
+    # not turn this screen amber. Same source the canvas marks nodes from.
+    unhealthy = controller.unhealthy_nodes()
     if quarantined:
         problems.append(f"{len(quarantined)} flow(s) quarantined")
     if errored: