Count the runs in flight on Home, where an installation has any

The tile only exists on an installation that has run something, which is the
same "nothing has been run yet" the Runs screen goes by. The count comes from
/runs/overview, already grouped by status in the database and already polled by
the Runs screen, so Home shares its cache entry rather than paging rows or
growing a second endpoint.

docs/interface/index.md

@@ -57,6 +57,8 @@ Always answers, degraded or not. The tiles cover:
 - **Flows** — total, running, paused, quarantined, and how many cannot run
   because their graph does not validate
 - **Nodes** — how many failed to load
+- **Runs running** — batch runs in flight right now, and how many are
+  waiting. Only on an installation that has run something
 - **Queue** — depth, and how old the oldest pending item is
 - **Loop lag** — whether the engine's event loop is keeping up