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>

crates/backend/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "backend"
+version = "0.3.0"
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+description = "IdeA shared backend composition root, reusable by desktop and server adapters."
+
+[dependencies]
+domain = { workspace = true }
+application = { workspace = true }
+infrastructure = { workspace = true }
+tokio = { workspace = true, features = ["io-std", "rt"] }
+serde = { workspace = true }
+serde_json = { workspace = true }
+uuid = { workspace = true }
+async-trait = { workspace = true }
+interprocess = { version = "2.4", features = ["tokio"] }
+
+[features]
+vector-http = ["infrastructure/vector-http"]
+vector-onnx = ["infrastructure/vector-onnx"]