feat(cmf): live-apply managed-server changes; auto-resolve import id collisions

- Saving the Local models list now applies IMMEDIATELY: removed or redefined
  rows stop their `cortiq serve` child (memory freed on the spot), new or
  changed rows are spawned and health-checked — no gateway restart needed.
- A managed row that shares its id with a same-id STATIC pool entry pointing at
  a local server (the shape Import→register creates) now supersedes it: the
  static entry is dropped automatically instead of failing the save with
  "duplicate managed model id".
- validate() now tells the two duplicate cases apart with actionable messages
  (two managed rows vs managed-vs-pool collision).

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cortiq-gateway"
-version = "0.2.19"
+version = "0.2.20"
 edition = "2021"
 rust-version = "1.75"
 license = "Apache-2.0"