Give each pending assignment its own condition, and keep a raised row until it clears

Second review of #150: one key per machine and module let a newer failure
be cleared in the tick that raised it, pruning could orphan an open
condition, and a build no longer waited for read as never asked although it
may still run.

internal/inventory/migrations/0082-an-assignment-waits-for-its-build.sql

@@ -8,8 +8,10 @@
 
 -- Every build the controller asked for: a merge's new module, a plan's tier, a person's `build`, an `assign`
 -- with build. Kept once the ask is made, never before. Its outcome is the `build` row of the same id;
--- an ask without one is still running, or was lost. not_asked is why the ask could not be made or waited
--- for; for a merge that could not ask, the id is the controller's and no build's.
+-- an ask without one is still running, or was lost. not_asked is why the ask could not be handed over; for a
+-- merge that could not ask, the id is the controller's and no build's. outcome_unknown is why an asker that
+-- handed it over stopped waiting: the build may still run, and is read as in flight until its outcome or
+-- its bound.
 create table build_request (
     id          text primary key,
     repository  text not null,
@@ -19,6 +21,7 @@ create table build_request (
     commit_hash text not null default '',
     asked_for   text not null default '',
     not_asked   text,
+    outcome_unknown text,
     asked_at    timestamptz not null default now()
 );
 create index build_request_asked_at on build_request (asked_at);
@@ -28,7 +31,7 @@ create index build_request_asked_at on build_request (asked_at);
 -- assignment was refused then), expired (the build failed, was not registered, or said nothing within its
 -- bound) or withdrawn (`unassign`). raised_at is when an expiry or refusal was raised as a condition,
 -- acknowledged_at when a person took it back with `unassign`, cleared_at when its condition was cleared.
--- Gone with its machine. Ended rows are deleted after 30 days.
+-- Gone with its machine. Ended rows are deleted after 30 days, once any condition raised for them is cleared.
 create table pending_assignment (
     id              bigserial primary key,
     node            uuid not null references node (id) on delete cascade,