refactor(backend): extraire le cœur backend commun hors Tauri (#13)

Lot B1 du chantier server/client mode : création de la crate `backend`
qui héberge le cœur commun (endpoint MCP, outils OpenAI) indépendant de
Tauri, préalable au futur serveur web + PTY WebSocket.

- crates/backend : nouvelle crate (lib.rs, mcp_endpoint.rs, openai_tools.rs).
- crates/app-tauri : câblage sur la crate backend (state.rs, mcp_bridge.rs,
  Cargo.toml, tests/orchestrator_wiring.rs).
- Cargo.toml / Cargo.lock racine : ajout de la crate au workspace.

Validé : cargo check --workspace vert, tests backend/app-tauri verts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.lock

@@ -73,6 +73,7 @@ version = "0.3.0"
 dependencies = [
  "application",
  "async-trait",
+ "backend",
  "base64 0.22.1",
  "domain",
  "infrastructure",
@@ -146,6 +147,21 @@ version = "1.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f2032f911046de80f0a198e0901378627c33f59ea0ac00e363d481118bd70a53"
 
+[[package]]
+name = "backend"
+version = "0.3.0"
+dependencies = [
+ "application",
+ "async-trait",
+ "domain",
+ "infrastructure",
+ "interprocess",
+ "serde",
+ "serde_json",
+ "tokio",
+ "uuid",
+]
+
 [[package]]
 name = "base64"
 version = "0.13.1"