feat(core): price a run by the size it holds, on hardware we pay for

The rate was a constant. That was correct for everything the system can
currently run and wrong the moment it can run anything else, because a tier
buys a share of a card — so a bigger one on our own hardware is more of
something we bought being spent, and a flat rate there sells a whole card for
the price of a quarter of one.

So a run's rate now comes from what the run is: its size tier, and whose
hardware it sits on.

On the caller's own hardware the tier changes nothing. There is no share of a
card of ours in play, so a run costs one unit a second whatever size it asked
for. Charging somebody more for taking more of a GPU they bought is a tax on
their own hardware, and not doing that is most of what this model is for.

This exposed a bug in what went before. Resegmenting recomputed one shared rate
and wrote it to every open stretch, which was harmless while all runs cost the
same and would have quietly repriced an expensive run as whatever the last one
to start was. Each stretch now keeps its own rate, which is also the more
honest shape: a run's rate is a property of that run, and nothing about it
changed because a sibling appeared or the clock ticked.

The account's total is now the sum of what its runs cost rather than a count
times one rate — an expensive run and a cheap one alongside it are not two of
anything. Concurrency still lands exactly where it did, as there being more to
add, and no run gets dearer because another started.

There is no hardware factor yet and its absence is deliberate: nothing records
which card a host has, so a table keyed on a model would be keyed on nothing.
A faster card should cost more, and that starts with a column.

The reference tier is pinned at exactly one unit a second, checked rather than
assumed. The unit is a second of a reference session, so moving it would
silently redefine every allowance — the same stored number would mean a
different number of hours.

apps/api/app/routes/session.ts

@@ -236,8 +236,14 @@ export namespace SessionApi {
 				// being discarded, because the caller has to be told what it will
 				// cost and what remains — and asking a second time would let the
 				// number shown and the number billed disagree.
-				const team = await Billing.teamForBox(box.id);
-				const billing = team ? await Billing.assertMayStart(team) : null;
+				const payer = await Billing.teamForBox(box.id);
+				const billing = payer
+					? await Billing.assertMayStart({
+							teamId: payer.teamId,
+							nextTier: payer.tier,
+							nextHostClass: payer.hostClass
+						})
+					: null;
 
 				const session = await Session.request({
 					id: Identifier.ascending('session'),