feat(backend): historique, SSE et Prometheus — Phase 6

- Migration 002 : table metrics_history (index agent_id/timestamp)
- AppState : broadcast channels (metrics_tx, network_tx) + FromRef<SqlitePool>
- GET /api/v1/history/{agent_id}?hours=N : série temporelle, rétention 7 jours
- POST /api/v1/metrics : écrit dans history + broadcast SSE
- GET /api/v1/stream : Server-Sent Events (events metrics + network)
- GET /metrics : endpoint Prometheus text/plain (CPU, RAM, disque, temp, réseau)
- Bump version API 0.2.0

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

backend/Cargo.toml

@@ -15,3 +15,5 @@ sqlx             = { version = "0.8", features = ["sqlite", "runtime-tokio", "ma
 tower-http       = { version = "0.6", features = ["cors", "trace"] }
 utoipa           = { version = "5", features = ["axum_extras"] }
 chrono             = { version = "0.4", features = ["serde"] }
+tokio-stream       = { version = "0.1", features = ["sync"] }
+futures            = "0.3"