[v0.21.x-branch] Backport #11206: invoices: bound rapid migration test workload

[github-actions]

Backport of #11206

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Change

TestMigrateSingleInvoiceRapid performed Rapid's default 100 property checks,
but generated and migrated another 100 invoices inside every check for both
SQLite and Postgres. The resulting 20,000 migrations and delayed database
cleanup made the race build exceed the Postgres fixture's fixed ten-minute
lifetime.

• Run SQLite and Postgres as separate subtests so the Postgres fixture lifetime
  covers only Postgres checks.
• Close each database when its Rapid property check finishes instead of
  retaining every handle until the outer test exits.
• Migrate a bounded batch of ten randomized invoices per property check. This
  retains multiple migrations in one transaction and post-commit lookup
  coverage while replacing the accidental 100-by-100 multiplier with 1,000
  generated invoices per backend.

This addresses the repeated race-job timeout observed after the Go 1.27.1
toolchain update in #11200 without increasing the fixture timeout.

Verification

• Full Go 1.27.1 race run of TestMigrateSingleInvoiceRapid: pass in 142.330s.
• Fixed-seed 20-check comparison: 30.884s, down from 65.900s.
• make lint-native: 0 issues.