fix(backend): wait for in-flight account deletion retry before mongo stop

[claude]

Fixes #3442

What and why

PostHog error tracking caught a single staging occurrence of Could not retry pending account deletions did you forget to call \start`?, thrown from MongoService#accessInternalCollectionProps(mongo.service.ts:144) viaUserService.retryPendingAccountDeletions(user.service.ts:354, thependingSyncPurge` query).

Root cause: gracefulShutdown in app.ts calls userService.stopAccountDeletionRetries() (which only clearIntervals) before mongoService.stop(). clearInterval stops future firings, but a retry cycle already in flight from the previous interval tick keeps running as a detached void ...catch(...) promise. If shutdown lands mid-cycle, mongoService.stop() clears #internalClient while that cycle is still awaiting a pendingAccountDeletion query, so the next collection-getter access throws "did you forget to call start?". This matches the observed stack trace exactly (first query in the cycle succeeded, the second one at line 354 hit the now-undefined client) and the event only occurred on staging, consistent with a deploy/restart race rather than steady-state traffic.

Fix: track the in-flight retry cycle's promise on UserService and make stopAccountDeletionRetries async, awaiting that promise (each cycle already catches and logs its own errors) before returning. app.ts now awaits it in gracefulShutdown, so mongoService.stop() never runs while a retry cycle is still using the client.

Verify

684 pass, 1 skip, 0 fail, 1643 expect() calls. Ran 685 tests across 74 files.

Selected packages: backend
Checks run: test:backend, type-check, lint, knip
Checks skipped: (none)

VERDICT: PASS

New regression test (account deletion retry lifecycle in user.service.db.test.ts) fails on the pre-fix code (stopAccountDeletionRetries returned void, so nothing to .then() on) and passes with the fix.

Independent review

Self-reviewed only; this PR was opened by the error-autofix pipeline (mode: merge). No second human/AI reviewer yet.

Test plan

• Added a regression test that reproduces the shutdown race (mocks retryPendingAccountDeletions to stay in flight, starts the retry timer, and asserts stopAccountDeletionRetries() doesn't resolve until the cycle finishes)
• Verified the new test fails against the pre-fix user.service.ts/app.ts and passes with the fix
• bun run verify --strict is green

[cursor]

Still relevant: the shutdown race is present on current main (stopAccountDeletionRetries still only clearIntervals, and gracefulShutdown does not await it). Merged latest main into this branch so the PR is up to date. Re-running bun run verify --strict next.

[cursor]

bun run verify --strict after merging latest main:

Selected packages: backend
Checks run: test:backend, type-check, lint, knip
Checks skipped: (none)

✓ All checks passed
VERDICT: PASS

The new regression test (UserService > account deletion retry lifecycle) passed. Enabled merge (repo merge queue / squash).