fix(uploads): stop a storage hold from being dropped by whoever can name it

A reservation id was never a secret and could not have been one. An upload
token is base64url(payload) followed by its signature, so a client can read
every claim out of its own token, and the two R2 init routes hand their
reservation ids to the client outright. The asset route takes a reservation
id from the request body and deleted it on the strength of that id and the
billed user alone, and every hold an account owns is billed to the same user.

So a caller could start a Bunny upload, read the id out of the token they
were just given, quote it while attaching a one byte image or even a bare
YouTube link, and have the quota handed back while the upload carried on.
Repeat and a trial worth three gigabytes uploads as much as it likes for as
long as Bunny takes to report a figure of its own. Signing the id rather than
handing it over bought nothing, because signing is not hiding.

A hold now records what it was opened for and is only ever consumed by that
flow, so naming one is no longer enough to drop it.

Guests hold against the workspace owner's quota rather than their own and had
no way to give it back: the release was gated on being signed in. Declaring a
size and walking away cost the guest nothing and cost the owner their whole
remaining allowance for two hours. The guest grant now carries the reservation
and the declared size, bound to the Bunny video as well as to ours, so
cancelling gives the quota back and costs them the upload it stood for. What a
guest can hold without cancelling lapses in half an hour rather than two hours.

The in-transaction fallback check counted the account's Bunny storage as zero
on a Bunny upload, because the figure was only prefetched for R2 providers and
that branch was unreachable for Bunny until this PR made it reachable. On an
account whose storage is all Bunny that was a check that could not fail. It is
prefetched for every provider that can reach the fallback now.

prisma/migrations/20260818120000_add_upload_reservation_purpose/migration.sql

@@ -0,0 +1,10 @@
+-- AlterTable: record what each hold was opened for, so a reservation can only be
+-- consumed by the flow that opened it. Without this a caller who could name a
+-- reservation id could drop any of their own holds through whichever finalize
+-- route was cheapest, which defeats the point of holding one at all.
+--
+-- Rows that predate this column get 'LEGACY', which matches no finalize route.
+-- Those uploads fall through to the in-transaction quota check instead, and the
+-- rows lapse on their own TTL within the hour.
+ALTER TABLE "upload_reservations" ADD COLUMN "purpose" TEXT NOT NULL DEFAULT 'LEGACY';
+ALTER TABLE "upload_reservations" ALTER COLUMN "purpose" DROP DEFAULT;