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/app-tauri/Cargo.toml

@@ -23,6 +23,7 @@ tauri-build = { workspace = true }
 domain = { workspace = true }
 application = { workspace = true }
 infrastructure = { workspace = true }
+backend = { workspace = true }
 tauri = { workspace = true }
 tauri-plugin-dialog = { workspace = true }
 # `io-std` (on top of the workspace features) gives the headless `mcp-server`
@@ -44,8 +45,8 @@ interprocess = { version = "2.4", features = ["tokio"] }
 [features]
 # Passthrough toggles to enable the real embedders in an IDE build. OFF by default
 # (founding posture: `none` ⇒ naïve recall, zero dependency).
-vector-http = ["infrastructure/vector-http"]
-vector-onnx = ["infrastructure/vector-onnx"]
+vector-http = ["infrastructure/vector-http", "backend/vector-http"]
+vector-onnx = ["infrastructure/vector-onnx", "backend/vector-onnx"]
 
 [dev-dependencies]
 uuid = { workspace = true }