pruneExpiredRecords's defensive ?? 0 arms have zero test coverage, unlike its sibling function

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Context

src/db/retention.ts's pruneExpiredRecords has two defensive ?? 0 fallback arms guarding against
D1 driver anomalies:

• Line 75: results.push({ ..., deleted: Number(row?.n ?? 0) }); (dry-run count path)
• Line 85: const changes = Number(result.meta?.changes ?? 0); (actual delete-loop path)

Neither arm has any direct test coverage in test/unit/retention.test.ts today.

The exact same defensive pattern exists on this file's sibling function, dedupeSignalSnapshots
(lines 151 and 161), and that sibling's two equivalent ?? 0 arms do have dedicated tests —
test/unit/retention.test.ts lines 236-264, explicitly titled around a "defensive ?? 0 arm" mocking
env.DB to return a row/meta shape that triggers the fallback.

This repo's Codecov patch gate is 99%+ of changed lines and branches — these two untested nullish-
coalescing branches in pruneExpiredRecords are exactly the kind of gap the gate is meant to catch,
and the fix pattern is already fully precedented in the same file for the sibling function.

Requirements

• Add tests for pruneExpiredRecords's two ?? 0 fallback arms (lines 75 and 85), following the
  exact same mocking approach dedupeSignalSnapshots's existing tests (lines 236-264) already use for
  the identical pattern — mock env.DB to return a row/meta shape where the relevant field is
  missing/null/undefined, and assert the function falls back to 0 rather than throwing or producing
  NaN.
• Do not modify pruneExpiredRecords's actual logic — this issue is scoped to adding missing test
  coverage for existing, already-correct defensive code.

Deliverables

• A new test (or two) in test/unit/retention.test.ts covering pruneExpiredRecords's dry-run
  row?.n ?? 0 fallback (line 75), mirroring dedupeSignalSnapshots's existing equivalent test.
• A new test (or two) covering pruneExpiredRecords's delete-loop result.meta?.changes ?? 0
  fallback (line 85), mirroring dedupeSignalSnapshots's existing equivalent test.

Test Coverage Requirements

This repo's Codecov patch gate is 99%+ of changed lines and branches (src/** is covered). Since this
issue is pure test-addition (no production code changes), the deliverable is the coverage: both
previously-uncovered ?? 0 branches in pruneExpiredRecords must show as covered after this PR.

Expected Outcome

pruneExpiredRecords's two defensive fallback arms have the same direct test coverage its sibling
dedupeSignalSnapshots already has for the identical pattern, closing a Codecov branch gap with no
production-code risk.

Links & Resources

• src/db/retention.ts:75,85 — the two branches to cover
• src/db/retention.ts:151,161 — dedupeSignalSnapshots's identical pattern
• test/unit/retention.test.ts:236-264 — the existing tests to mirror

[linear-code]

JSO-5874