feat(core): a box is a row, a session is the billing unit

Migration 1 of 0048, and the first of the seven weeks — nothing about a live
feed works without these two tables, so it is not a cleanup during them.

  box      a VM someone owns: an id that is also its DNS label, an editable
           label, an owning user, the machine it sits on, a tier and a state.
           Owned by a person and placed on a team's hardware, which are two
           different relationships, hence both userId and machineId.
  session  one run of one box by one linked Steam account, and what costs
           money. Separate from box because the ticket changes after bind as
           addresses are discovered — the vsock contract calls it "a stream,
           not one value" — so it is a column a client polls, not a value it
           is handed once.

Box states are neslet's own three and no more. `starting` and `stopping` are
the obvious additions and both are omitted because nothing would ever write
them; a failed box is `stopped` with stopClean false, which is how neslet
models it too.

The generated migration would have failed on live rows in three ways, so it
is hand-written and tested against a database seeded at the old schema:

  - machine.team_id becomes notNull, and *every existing row is null* because
    the old registration path passed null. Personal teams are backfilled for
    machine owners first, reusing a team they already own rather than minting
    a second, with the owner membership row repaired where missing.
  - game_download.host_id becomes a foreign key. It held free-form strings,
    so unattributable rows are deleted before the cast — the only destructive
    statement here, and a considered loss: it is a progress report neslet
    re-derives from disk.
  - Team.createPersonal was written and documented in packages/core/CLAUDE.md
    as part of the login flow and never actually called, so no user has a
    team. ensurePersonal is idempotent and now runs on every login, which is
    what backfills accounts the migration does not reach.

Verified on a seeded legacy database: three null-team machines backfilled, an
existing team reused rather than duplicated, a blank display name handled, and
both unattributable download rows dropped while the attributable one survived.

Also fixes two things this work ran into rather than caused:

  - Database.client() built a new postgres pool on every call, and use()
    called it twice per invocation — pools of ten connections held for a 30s
    idle timeout. Invisible in a Worker where requests are short; the suite
    crossed 100 connections and Postgres said "sorry, too many clients
    already" in whichever file ran last, which reads as a flaky test rather
    than a leak. Now one pool per connection string.
  - download.test.ts asserted against `hst_…` host ids, which is exactly the
    unattributable row the new foreign key exists to refuse.

There is no "no team" any more: PATCH /machine/:id took teamId null to mean
"mine alone" and now requires a team, because the personal team is the one to
name. Its test is updated to the new contract rather than deleted.

113 → 128 tests, 0 fail.

packages/core/src/machine/index.ts

@@ -32,8 +32,9 @@ export namespace Machine {
 				description: 'The user who registered this machine',
 				example: Examples.Machine.ownerUserId
 			}),
-			teamId: z.string().optional().nullable().meta({
-				description: 'The team this machine belongs to, when registered inside one',
+			teamId: z.string().meta({
+				description:
+					'The team that owns this hardware. Always set — every user has a personal team',
 				example: Examples.Machine.teamId
 			}),
 			label: z.string().meta({
@@ -89,7 +90,7 @@ export namespace Machine {
 				await tx.insert(MachineTable).values({
 					id: input.id,
 					ownerUserId: input.ownerUserId,
-					teamId: input.teamId ?? null,
+					teamId: input.teamId,
 					label: input.label,
 					secretHash: await hashSecret(secret),
 					lastSeen: null
@@ -130,7 +131,13 @@ export namespace Machine {
 	);
 
 	/**
-	 * Move a box into a team, or back out of one with `teamId: null`.
+	 * Move a host to a different team.
+	 *
+	 * There is no "out of a team" any more: `teamId` is notNull since
+	 * [0048](../../../../.nestri/decisions/0048-email-is-the-root-identity-and-a-box-is-a-row.md),
+	 * so a host always belongs to exactly one, and the single-operator case is a
+	 * team of one rather than a null. What used to be *unscope* is now *move to
+	 * my personal team*, which the caller names explicitly.
 	 *
 	 * Scoped to the owner in the query itself, so a machine belonging to
 	 * someone else is a miss rather than a permission check that could be
@@ -147,7 +154,7 @@ export namespace Machine {
 			return Database.use(async (tx) => {
 				return tx
 					.update(MachineTable)
-					.set({ teamId: input.teamId ?? null })
+					.set({ teamId: input.teamId })
 					.where(
 						and(
 							eq(MachineTable.id, input.id),