Issue #3614328: Rename a tier's capacity to quota, because it is not a capacity

A capacity is how many a space holds. A tier's number is a selling allocation an operator typed, and it may be any number at all, including one larger than the room. Naming both capacity made a booking page reading "825 left" for one tariff, in a hall with 813 places of that grade, look like a bug when nothing was broken: the page prints whichever of the two is lower.

What changes

• yoyaku_slot_category.capacity becomes quota; getCapacity() becomes getQuota().
• yoyaku_category.default_capacity becomes default_quota; getDefaultCapacity() becomes getDefaultQuota().
• Column names follow. Pre-1.0, so no update hook and no compatibility shim: reinstall is the upgrade path.

What deliberately does not change

A slot's capacity, a resource's capacity and a pooled area's pool_capacity all describe space rather than an allocation, so they keep the word, as does CapacityExceededException. BookingManager::limitFor() carried both concepts in one line and now reads getQuota() on the tier side and getCapacity() on the slot side.

Note on the rename hazard

Three classes expose an identically named getCapacity() (resource, slot, slot category) and only one of them renames, so no search and replace is safe. The call sites were enumerated with phpstan at level 2; the CI ruleset runs at level 1, which does not report them.

Wording

An empty tier value no longer reads "Unlimited" but "No quota", because a tier without a quota is not unlimited: where the resource runs in a venue the free places of the grades it prices bound it, and the lower of the two wins. Docs and the French translations ship in the same commit.