feat: preserve source scanner for synced devices

When devices are synced from a peer, they now retain their original
source scanner reference. This ensures:

- Device positions are calculated relative to the scanner that
  detected them, not the local scanner
- Moving the local scanner won't affect synced devices' positions
- Popup shows "Source: <scanner_id>" for remotely-synced devices

Database changes:
- Added source_scanner_id, source_scanner_lat, source_scanner_lon
  columns to devices table
- get_devices_since() includes source scanner info in sync data
- bulk_update_devices() accepts and stores source scanner position
- Added get_all_device_sources() method

API changes:
- /api/sync/devices GET includes scanner_lat and scanner_lon
- /api/sync/devices POST accepts source_scanner_lat/lon
- /api/device/floors includes sources dict

Frontend changes:
- loadDevicePositions() loads source scanner info
- getDevicePosition() uses source scanner position for synced devices
- Popup shows source scanner info for remotely-synced devices

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

src/rf_mapper/sync.py

@@ -124,7 +124,15 @@ class PeerSync:
         devices = data.get("devices", [])
         source_scanner = data.get("scanner_id", "unknown")
 
-        updated = self.db.bulk_update_devices(devices, source_scanner)
+        # Get source scanner position for correct device positioning
+        source_lat = data.get("scanner_lat")
+        source_lon = data.get("scanner_lon")
+
+        updated = self.db.bulk_update_devices(
+            devices, source_scanner,
+            source_scanner_lat=source_lat,
+            source_scanner_lon=source_lon
+        )
         return updated
 
     def push_devices_to_peer(self, peer_url: str, since: Optional[str] = None) -> dict:
@@ -141,6 +149,8 @@ class PeerSync:
 
         payload = {
             "source_scanner": self.scanner_identity["id"],
+            "source_scanner_lat": self.scanner_identity["latitude"],
+            "source_scanner_lon": self.scanner_identity["longitude"],
             "devices": devices
         }