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>

crates/backend/src/events.rs

@@ -119,6 +119,60 @@ pub enum DomainEventDto {
         /// Project id (UUID string).
         project_id: String,
     },
+    /// A plugin was installed.
+    #[serde(rename_all = "camelCase")]
+    PluginInstalled {
+        /// Plugin id.
+        plugin_id: String,
+        /// Installed version.
+        version: String,
+    },
+    /// A plugin was enabled.
+    #[serde(rename_all = "camelCase")]
+    PluginEnabled {
+        /// Plugin id.
+        plugin_id: String,
+    },
+    /// A plugin was disabled.
+    #[serde(rename_all = "camelCase")]
+    PluginDisabled {
+        /// Plugin id.
+        plugin_id: String,
+        /// Whether a restart is needed for full JS purge.
+        restart_required: bool,
+    },
+    /// A plugin was uninstalled.
+    #[serde(rename_all = "camelCase")]
+    PluginUninstalled {
+        /// Plugin id.
+        plugin_id: String,
+        /// Whether a restart is needed for full JS purge.
+        restart_required: bool,
+    },
+    /// A plugin failed to load.
+    #[serde(rename_all = "camelCase")]
+    PluginLoadFailed {
+        /// Plugin id.
+        plugin_id: String,
+        /// Failure reason.
+        reason: String,
+    },
+    /// A plugin MCP server started.
+    #[serde(rename_all = "camelCase")]
+    PluginMcpServerStarted {
+        /// Plugin id.
+        plugin_id: String,
+        /// Server id.
+        server_id: String,
+    },
+    /// A plugin MCP server stopped.
+    #[serde(rename_all = "camelCase")]
+    PluginMcpServerStopped {
+        /// Plugin id.
+        plugin_id: String,
+        /// Server id.
+        server_id: String,
+    },
     /// An agent was launched.
     #[serde(rename_all = "camelCase")]
     AgentLaunched {
@@ -677,6 +731,45 @@ impl From<&DomainEvent> for DomainEventDto {
             DomainEvent::ProjectCreated { project_id } => Self::ProjectCreated {
                 project_id: project_id.to_string(),
             },
+            DomainEvent::PluginInstalled { plugin_id, version } => Self::PluginInstalled {
+                plugin_id: plugin_id.to_string(),
+                version: version.as_str().to_owned(),
+            },
+            DomainEvent::PluginEnabled { plugin_id } => Self::PluginEnabled {
+                plugin_id: plugin_id.to_string(),
+            },
+            DomainEvent::PluginDisabled {
+                plugin_id,
+                restart_required,
+            } => Self::PluginDisabled {
+                plugin_id: plugin_id.to_string(),
+                restart_required: *restart_required,
+            },
+            DomainEvent::PluginUninstalled {
+                plugin_id,
+                restart_required,
+            } => Self::PluginUninstalled {
+                plugin_id: plugin_id.to_string(),
+                restart_required: *restart_required,
+            },
+            DomainEvent::PluginLoadFailed { plugin_id, reason } => Self::PluginLoadFailed {
+                plugin_id: plugin_id.to_string(),
+                reason: reason.clone(),
+            },
+            DomainEvent::PluginMcpServerStarted {
+                plugin_id,
+                server_id,
+            } => Self::PluginMcpServerStarted {
+                plugin_id: plugin_id.to_string(),
+                server_id: server_id.as_str().to_owned(),
+            },
+            DomainEvent::PluginMcpServerStopped {
+                plugin_id,
+                server_id,
+            } => Self::PluginMcpServerStopped {
+                plugin_id: plugin_id.to_string(),
+                server_id: server_id.as_str().to_owned(),
+            },
             DomainEvent::AgentLaunched {
                 agent_id,
                 session_id,