feat(backend): implémentation complète Phase 1

- Migration SQLite initiale : agents, devices, metrics, events
- API REST v1 complète : /agents, /network, /metrics, /events, /widgets
- Endpoints widgets Glance : /api/v1/widgets/network et /api/v1/widgets/metrics
- Spec OpenAPI générée et servie sur /api-docs/openapi.json
- Gestion d'erreurs centralisée (AppError)
- CORS permissif pour développement
- push réseau met à jour le last_seen de l'agent

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

backend/Cargo.toml

@@ -13,5 +13,5 @@ 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"] }
+utoipa           = { version = "5", features = ["axum_extras"] }
+chrono             = { version = "0.4", features = ["serde"] }