An item shared through the sharing tier is visible to its recipients but
carried by its owner. The bag totals filtered on bag_id alone, so every
recipient saw the owner's weight added to their own load — which is exactly
the situation that tier exists for (one person brings the first-aid kit).
Filtering by owner_id, as the issue suggests, would have broken more than it
fixed: owner_id is stamped on every item including Common ones, so the group
pool would have shrunk to "only what I entered myself" even on trips where
nobody shares anything. The predicate keeps the Common pool intact and drops
only private items owned by someone else.
Closes#1767
Maurice
446bbdc38f