fix: GPS never from seed — only from real check-ins

GPS (latitude/longitude) and photo_path are now NEVER set by seed
import pipelines, even if empty. GPS comes only from field tech
check-ins via the main app.

- db_writer.py: FIELDS_NEVER_FROM_SEED set, GPS always None in
  _extract_row_values, skipped in update mode
- sync.py: latitude/longitude set to None in asset_data
- FIELD_MAP.md: updated policy table

sync.py

@@ -200,8 +200,8 @@ def push(source_db: str, target_db: str, dry_run: bool = False) -> dict:
                 "customer_id": customer_id,
                 "location_id": location_id,
                 "assigned_to": None,
-                "latitude": _val(r, "latitude"),
-                "longitude": _val(r, "longitude"),
+                "latitude": None,    # GPS comes only from real check-ins
+                "longitude": None,   # GPS comes only from real check-ins
                 "geofence_radius_meters": 50,
                 "disney_park": _val(r, "disney_park"),
                 "is_disney": is_disney,