Prober: claim only candidates of settled releases, without the join

The settler marks a settled release's candidates ready (and unreadable when
it had no readable name). Claims took the candidates by joining releases
from random points of the queue; in its newer half nothing had settled, so
a slice scanned to the end (0.3 s each, 20 slices and 5 tiers a claim) and
the prober stood still: 2.4M waiting, 0 probed a minute. Now an index over
the claimable rows only, the slices drawn within them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

database/schema.ts

@@ -169,7 +169,7 @@ export class ProbeCacheSchema extends BaseModel {
 }
 
 export class ProbeCandidateSchema extends BaseModel {
-  static $columns = ['createdAt', 'file', 'id', 'kind', 'messageId', 'releaseId', 'status'] as const
+  static $columns = ['createdAt', 'file', 'id', 'kind', 'messageId', 'ready', 'releaseId', 'status', 'unreadable'] as const
   $columns = ProbeCandidateSchema.$columns
   @column.dateTime({ autoCreate: true })
   declare createdAt: DateTime
@@ -182,9 +182,13 @@ export class ProbeCandidateSchema extends BaseModel {
   @column()
   declare messageId: string
   @column()
+  declare ready: boolean
+  @column()
   declare releaseId: bigint | number
   @column()
   declare status: string
+  @column()
+  declare unreadable: boolean
 }
 
 export class RangeSchema extends BaseModel {