fix(fleet): name the BACKEND in a placement refusal, not just "microvm capability"

Observed on the negative control: a mission with backend='kimi' was correctly
refused, but the message read "no online node reports microvm capability" — and
both nodes do report it. What one lacked was the image. That first clause would
have sent an operator to reinstall firecracker on a node that already had it.

The refusal now names the backend, and the remedy still names both halves.

crates/cm-api/src/mission_orchestrator.rs

@@ -152,10 +152,19 @@ pub async fn on_launch(
             Some(want) => *capable
                 .iter()
                 .find(|n| n.as_uuid() == want)
-                .ok_or_else(|| format!("mission targets node {want}, which is not an online node reporting microvm capability ({how_to_fix})"))?,
+                .ok_or_else(|| format!(
-            None => *capable
+                    "mission targets node {want}, which is not an online node that can \
-                .first()
+                     run backend {:?} — {how_to_fix}",
-                .ok_or_else(|| format!("no online node reports microvm capability ({how_to_fix})"))?,
+                    backend.unwrap_or("default")
+                ))?,
+            None => *capable.first().ok_or_else(|| format!(
+                // Names the BACKEND, not just "microvm capability". Both nodes
+                // report that capability; what one of them lacked was the image.
+                // The first version of this message would have sent an operator to
+                // reinstall firecracker on a node that already had it.
+                "no online node can run backend {:?} — {how_to_fix}",
+                backend.unwrap_or("default")
+            ))?,
         };
         sqlx::query("UPDATE missions SET target_node_id = $1, updated_at = now() WHERE id = $2")
             .bind(chosen.as_uuid())