fix(missions): a microvm mission could not be created at all

`runtime_kind='microvm'` passes the DB CHECK, is honoured by placement, and now
has an executor — but `POST /api/missions` rejected the value with 400, so the
only interface that creates missions could not produce one. And `backend`, which
selects the per-CLI rootfs, was not in the create payload at all: it existed as a
column and as a parameter to `vm_create`, with nothing able to set it.

microvm needs no target_node_id at create time, unlike local_herdr: placement
resolves a KVM-capable node at launch and fails the launch when there is none, so
an explicit target is a request rather than a requirement.

461 tests pass, clippy clean.

Co-Authored-By: Claude Opus 5 <[email protected]>

crates/cm-api/src/routes/missions.rs

@@ -38,6 +38,9 @@ pub struct CreateMissionRequest {
     /// Defaults to "zeroclaw". "local_herdr" requires target_node_id.
     pub runtime_kind: Option<String>,
     pub target_node_id: Option<Uuid>,
+    /// Which per-CLI rootfs a `microvm` mission boots (`missions.backend`), e.g.
+    /// "claude". NULL boots the node's default image.
+    pub backend: Option<String>,
 }
 
 fn default_schedule() -> Value {
@@ -260,6 +263,12 @@ pub async fn create(
                 return Err(ApiError::BadRequest);
             }
         }
+        // microvm needs no target here: placement resolves a KVM-capable node at
+        // launch and fails the launch when there is none, so an explicit target is
+        // a request rather than a requirement. Rejecting the value outright — as
+        // this did until B4.5 — made `runtime_kind='microvm'` unreachable through
+        // the only interface that creates missions.
+        "microvm" => {}
         _ => return Err(ApiError::BadRequest),
     }
 
@@ -275,6 +284,7 @@ pub async fn create(
         config: body.config,
         runtime_kind: Some(runtime_kind),
         target_node_id: body.target_node_id,
+        backend: body.backend.as_deref(),
         phases: phases_for_create(
             crate::workflow_registry::get(body.template_kind.trim()),
             body.phases,