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.toml

@@ -4,6 +4,7 @@ members = [
     "crates/domain",
     "crates/application",
     "crates/infrastructure",
+    "crates/backend",
     "crates/app-tauri",
 ]
 
@@ -29,6 +30,7 @@ git2 = { version = "0.20", default-features = false }
 domain = { path = "crates/domain" }
 application = { path = "crates/application" }
 infrastructure = { path = "crates/infrastructure" }
+backend = { path = "crates/backend" }
 
 # Tauri v2
 tauri = { version = "2", features = [] }