chore: initialise la structure du projet SentinelMesh

- Workspace Cargo avec backend, agent-scan-network, agent-metric
- Skeleton Rust pour les trois crates (Axum, Tokio, SQLx)
- Documentation : README, FEATURES, ROADMAP, ARCHITECTURE, API, INSTALL
- Exemples de widgets Glance (custom-api)
- Script d'installation agents (squelette Phase 5)
- Docker Compose + Dockerfile backend
- .gitignore et CLAUDE.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name    = "sentinelmesh-backend"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+axum             = "0.8"
+tokio            = { workspace = true }
+serde            = { workspace = true }
+serde_json       = { workspace = true }
+anyhow           = { workspace = true }
+tracing          = { workspace = true }
+tracing-subscriber = { workspace = true }
+sqlx             = { version = "0.8", features = ["sqlite", "runtime-tokio", "macros", "migrate"] }
+tower-http       = { version = "0.6", features = ["cors", "trace"] }
+utoipa           = { version = "4", features = ["axum_extras"] }
+utoipa-swagger-ui = { version = "7", features = ["axum"] }