feat(backend): système de plugins — domaine, application, infrastructure (#43)

Lots B1-B4 : modèle de domaine des plugins (manifeste, capacités menus/layouts/MCP),
port et registre applicatif, chargement/validation en infrastructure, exposition DTO
et commandes Tauri. Tests cargo verts.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Cargo.lock

@@ -1971,6 +1971,7 @@ dependencies = [
  "fastembed",
  "futures-util",
  "git2",
+ "hex",
  "landlock",
  "notify",
  "portable-pty",
@@ -1978,6 +1979,7 @@ dependencies = [
  "reqwest 0.12.28",
  "serde",
  "serde_json",
+ "sha2",
  "thiserror 2.0.18",
  "tokio",
  "uuid",