feat: add multi-scanner trilateration and signal heat map

- Add database methods for multi-scanner RSSI queries
- Add weighted trilateration function supporting 2+ scanners
- Add /api/positions/trilaterated endpoint
- Add /api/heatmap/signal endpoint for heat map data
- Update frontend to show trilaterated positions with gold markers
- Add heat map toggle button for signal coverage visualization

Trilateration uses RSSI from multiple scanners to calculate device
positions with confidence scores. Devices seen by 2+ scanners within
60 seconds get trilaterated positions shown with gold border markers.

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

src/rf_mapper/database.py

@@ -496,6 +496,56 @@ class DeviceDatabase:
             max_distance_m=round(stats['max_distance_m'], 2) if stats['max_distance_m'] else 0
         )
 
+    def get_device_multi_scanner_rssi(self, device_id: str, seconds: int = 60) -> list[dict]:
+        """Get recent RSSI readings per scanner for a device.
+
+        Args:
+            device_id: The device MAC address
+            seconds: Time window in seconds (default 60)
+
+        Returns:
+            List of dicts with scanner_id, avg_rssi, sample_count, last_seen
+        """
+        conn = self._get_connection()
+        cursor = conn.cursor()
+
+        query = """
+            SELECT scanner_id,
+                   AVG(rssi) as avg_rssi,
+                   COUNT(*) as sample_count,
+                   MAX(timestamp) as last_seen
+            FROM rssi_history
+            WHERE device_id = ?
+              AND scanner_id IS NOT NULL
+              AND timestamp >= datetime('now', '-' || ? || ' seconds')
+            GROUP BY scanner_id
+        """
+        cursor.execute(query, (device_id, seconds))
+        return [dict(r) for r in cursor.fetchall()]
+
+    def get_devices_seen_by_multiple_scanners(self, seconds: int = 60) -> list[str]:
+        """Get device IDs seen by 2+ scanners recently.
+
+        Args:
+            seconds: Time window in seconds (default 60)
+
+        Returns:
+            List of device IDs seen by multiple scanners
+        """
+        conn = self._get_connection()
+        cursor = conn.cursor()
+
+        query = """
+            SELECT device_id
+            FROM rssi_history
+            WHERE scanner_id IS NOT NULL
+              AND timestamp >= datetime('now', '-' || ? || ' seconds')
+            GROUP BY device_id
+            HAVING COUNT(DISTINCT scanner_id) >= 2
+        """
+        cursor.execute(query, (seconds,))
+        return [r['device_id'] for r in cursor.fetchall()]
+
     def get_movement_events(self, device_id: Optional[str] = None,
                             since: Optional[str] = None,
                             limit: int = 100) -> list[dict]: