feat: add /api/health endpoint for monitoring

- Returns status, version, uptime, scanner_id
- Component status: database, peer_sync, auto_scanner
- Returns 200 for healthy, 503 for unhealthy
- Tracks app start time for uptime calculation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/API.md

@@ -6,6 +6,65 @@ REST API documentation for RF Mapper web interface.
 
 ---
 
+## System
+
+### Health Check
+
+Returns health status for monitoring and load balancers.
+
+```
+GET /api/health
+```
+
+**Response:**
+
+```json
+{
+  "status": "healthy",
+  "version": "1.0.0",
+  "uptime_seconds": 3600,
+  "uptime_human": "1h 0m",
+  "scanner_id": "rpios",
+  "components": {
+    "database": {
+      "status": "ok",
+      "device_count": 100
+    },
+    "peer_sync": {
+      "status": "ok",
+      "peer_count": 2
+    },
+    "auto_scanner": {
+      "status": "stopped"
+    }
+  }
+}
+```
+
+**Status Codes:**
+- `200` - Healthy
+- `503` - Unhealthy (component error)
+
+**Component Status Values:**
+- `ok` - Component working normally
+- `disabled` - Component not enabled in config
+- `error` - Component has errors
+- `running` / `stopped` - For auto_scanner
+
+**Example:**
+
+```bash
+# Simple health check
+curl -s http://localhost:5000/api/health | jq '.status'
+
+# Use in monitoring scripts
+if curl -sf http://localhost:5000/api/health > /dev/null; then
+  echo "RF Mapper is healthy"
+fi
+```
+
+---
+
 ## Scanning
 
 ### Trigger Scan