FindProductSummary and FindColorDetail aggregate pan.position via GROUP_CONCAT
with a LEFT JOIN. When a product has no pans the join produces all-NULL rows,
causing GROUP_CONCAT to return NULL, which cannot be scanned into a Go string —
making the entire summary query fail silently and the frontend show an empty table.
Fix: wrap both GROUP_CONCAT(pan.position) calls with COALESCE(..., '').
Also removes CostPrice from the generated InvProduct model (field dropped from schema).
Adds integration tests TestFindProductSummaryNoPans and TestFindColorDetailNoPans
to guard against this class of NULL-scan regression.
