Health: count only flows the engine acts on, format durations with their unit

The "running" tile counted paused and invalid flows as running, so it read
"5/5" beside "1 flow(s) cannot run". Its note is now additive rather than a
precedence chain, so a quarantine no longer hides the invalid count.

Adds dur() beside si(): a ms reading picks its own unit, so a slow run reads
"1.24 s" instead of "1.2k ms".

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HTsT1isxUjw5gtkJk8WhuA

backend/app/api/routes/observability.py

@@ -157,8 +157,18 @@ async def read_summary(
         problems=problems,
         flows={
             "total": len(names),
+            # What the engine will actually act on. Enabled is not enough:
+            # validation stops a flow as surely as quarantine does, and a
+            # paused one is holding its messages rather than running them.
             "running": len(
-                [n for n in names if controller.is_enabled(n) and n not in quarantined]
+                [
+                    n
+                    for n in names
+                    if controller.is_enabled(n)
+                    and n not in quarantined
+                    and n not in paused
+                    and n not in invalid
+                ]
             ),
             "paused": len(paused),
             "quarantined": len(quarantined),