fix(sensing-server): detect ESP32 offline after 5s frame timeout (#300)

The source field was set to "esp32" on the first UDP frame but never
reverted when frames stopped arriving. This caused the UI to show
"Real hardware connected" indefinitely after powering off all nodes.

Changes:
- Add last_esp32_frame timestamp to AppStateInner
- Add effective_source() method with 5-second timeout
- Source becomes "esp32:offline" when no frames received within 5s
- Health endpoint shows "degraded" instead of "healthy" when offline
- All 6 status/health/info API endpoints use effective_source()

Fixes #297

Co-authored-by: Reuven <cohen@ruv-mac-mini.local>

rust-port/wifi-densepose-rs/crates/wifi-densepose-sensing-server/src/main.rs

@@ -285,6 +285,8 @@ struct AppStateInner {
     frame_history: VecDeque<Vec<f64>>,
     tick: u64,
     source: String,
+    /// Instant of the last ESP32 UDP frame received (for offline detection).
+    last_esp32_frame: Option<std::time::Instant>,
     tx: broadcast::Sender<String>,
     total_detections: u64,
     start_time: std::time::Instant,
@@ -364,6 +366,25 @@ struct AppStateInner {
     adaptive_model: Option<adaptive_classifier::AdaptiveModel>,
 }
 
+/// If no ESP32 frame arrives within this duration, source reverts to offline.
+const ESP32_OFFLINE_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(5);
+
+impl AppStateInner {
+    /// Return the effective data source, accounting for ESP32 frame timeout.
+    /// If the source is "esp32" but no frame has arrived in 5 seconds, returns
+    /// "esp32:offline" so the UI can distinguish active vs stale connections.
+    fn effective_source(&self) -> String {
+        if self.source == "esp32" {
+            if let Some(last) = self.last_esp32_frame {
+                if last.elapsed() > ESP32_OFFLINE_TIMEOUT {
+                    return "esp32:offline".to_string();
+                }
+            }
+        }
+        self.source.clone()
+    }
+}
+
 /// Number of frames retained in `frame_history` for temporal analysis.
 /// At 500 ms ticks this covers ~50 seconds; at 100 ms ticks ~10 seconds.
 const FRAME_HISTORY_CAPACITY: usize = 100;
@@ -1669,7 +1690,7 @@ async fn health(State(state): State<SharedState>) -> Json<serde_json::Value> {
     let s = state.read().await;
     Json(serde_json::json!({
         "status": "ok",
-        "source": s.source,
+        "source": s.effective_source(),
         "tick": s.tick,
         "clients": s.tx.receiver_count(),
     }))
@@ -1977,7 +1998,7 @@ async fn health_ready(State(state): State<SharedState>) -> Json<serde_json::Valu
     let s = state.read().await;
     Json(serde_json::json!({
         "status": "ready",
-        "source": s.source,
+        "source": s.effective_source(),
     }))
 }
 
@@ -1988,7 +2009,10 @@ async fn health_system(State(state): State<SharedState>) -> Json<serde_json::Val
         "status": "healthy",
         "components": {
             "api": { "status": "healthy", "message": "Rust Axum server" },
-            "hardware": { "status": "healthy", "message": format!("Source: {}", s.source) },
+            "hardware": {
+                "status": if s.effective_source().ends_with(":offline") { "degraded" } else { "healthy" },
+                "message": format!("Source: {}", s.effective_source())
+            },
             "pose": { "status": "healthy", "message": "WiFi-derived pose estimation" },
             "stream": { "status": if s.tx.receiver_count() > 0 { "healthy" } else { "idle" },
                         "message": format!("{} client(s)", s.tx.receiver_count()) },
@@ -2028,7 +2052,7 @@ async fn api_info(State(state): State<SharedState>) -> Json<serde_json::Value> {
         "version": env!("CARGO_PKG_VERSION"),
         "environment": "production",
         "backend": "rust",
-        "source": s.source,
+        "source": s.effective_source(),
         "features": {
             "wifi_sensing": true,
             "pose_estimation": true,
@@ -2049,7 +2073,7 @@ async fn pose_current(State(state): State<SharedState>) -> Json<serde_json::Valu
         "timestamp": chrono::Utc::now().timestamp_millis() as f64 / 1000.0,
         "persons": persons,
         "total_persons": persons.len(),
-        "source": s.source,
+        "source": s.effective_source(),
     }))
 }
 
@@ -2059,7 +2083,7 @@ async fn pose_stats(State(state): State<SharedState>) -> Json<serde_json::Value>
         "total_detections": s.total_detections,
         "average_confidence": 0.87,
         "frames_processed": s.tick,
-        "source": s.source,
+        "source": s.effective_source(),
     }))
 }
 
@@ -2083,7 +2107,7 @@ async fn stream_status(State(state): State<SharedState>) -> Json<serde_json::Val
         "active": true,
         "clients": s.tx.receiver_count(),
         "fps": if s.tick > 1 { 10u64 } else { 0u64 },
-        "source": s.source,
+        "source": s.effective_source(),
     }))
 }
 
@@ -2619,7 +2643,7 @@ async fn vital_signs_endpoint(State(state): State<SharedState>) -> Json<serde_js
             "heartbeat_samples": hb_len,
             "heartbeat_capacity": hb_cap,
         },
-        "source": s.source,
+        "source": s.effective_source(),
         "tick": s.tick,
     }))
 }
@@ -2825,6 +2849,7 @@ async fn udp_receiver_task(state: SharedState, udp_port: u16) {
 
                     let mut s = state.write().await;
                     s.source = "esp32".to_string();
+                    s.last_esp32_frame = Some(std::time::Instant::now());
 
                     // Append current amplitudes to history before extracting features so
                     // that temporal analysis includes the most recent frame.
@@ -3607,6 +3632,7 @@ async fn main() {
         frame_history: VecDeque::new(),
         tick: 0,
         source: source.into(),
+        last_esp32_frame: None,
         tx,
         total_detections: 0,
         start_time: std::time::Instant::now(),
@@ -3781,7 +3807,7 @@ async fn main() {
             "WiFi DensePose sensing model state",
         );
         builder.add_metadata(&serde_json::json!({
-            "source": s.source,
+            "source": s.effective_source(),
             "total_ticks": s.tick,
             "total_detections": s.total_detections,
             "uptime_secs": s.start_time.elapsed().as_secs(),