Committing a reservation returns stock to available instead of marking it sold

This is a revenue-accounting bug: a customer completes checkout, Commit reports success, and yet the units they just paid for silently become available for someone else to buy -- while sold (the figure that presumably drives revenue/units-sold reporting) never moves at all. Depending on which of two concurrent buyers' request wins the race for that "freed" stock, this can mean double-selling a physical unit that was already promised to someone else, and it permanently understates sales in any dashboard built on sold.

After committing a reservation, available goes back up by the committed quantity and sold never increases -- the committed stock is treated exactly like a released one.

1. go test ./tickets/tests/... -run TestTicket13 -v

applyStockEffect shares one function across commit, release, and expire because release and expire genuinely do the same thing (reserved -= qty, available += qty). Commit needs different SQL (reserved -= qty, sold += qty) -- but its case in the switch has been copy-pasted from the release/expire case instead, so it runs the exact same statement they do.

Look at the two SQL strings inside applyStockEffect's switch reason in pkg/inventory/service.go, and compare them against Commit's own doc comment right above it, which already states the correct effect: "reserved -= qty, sold += qty".