availableLanguages: videoChildren worker — additive sync to true replace

[tanflem]

Parent

Sub-issue of #9515 (Rebuild Video.availableLanguages as one cycle-safe, recursively-correct source of truth).

What to build

The worker that syncs the children relation from the childIds scalar array is changed from additive-only to a true replace, so a relation edge removed from childIds by any write path can't persist forever as a stale connection. This is independent of the recompute engine — it's a data-integrity hardening of the relation the cascade walks, not a change to the cascade itself.

Acceptance criteria

• A video whose childIds shrinks (a child id removed) has that child correctly disconnected from the children relation after the worker runs, not just new children connected.
• A video whose childIds grows continues to get the new child correctly connected (no regression).
• Running the worker twice in a row against unchanged data is a no-op (idempotent).

Blocked by

None (can start immediately) — independent of the recompute engine work.

[tanflem]

🔒 Phoebe holds a lease on this issue — runtime 23d237d8-ffd4-4f4e-960e-7682f69cf890, branch phoebe/bot-0-issue-9522.

This lease is refreshed periodically while work is in progress. If the heartbeat goes stale (the runtime died) or this runtime restarts, Phoebe reclaims the issue back to the ready queue automatically — no manual relabel needed.

[tanflem]

Addressed by Phoebe: #9528

[tanflem]

🔒 Mergetron holds a lease on this issue — runtime 23d237d8-ffd4-4f4e-960e-7682f69cf890, branch phoebe/bot-0-issue-9522.

This lease is refreshed periodically while work is in progress. If the heartbeat goes stale (the runtime died) or this runtime restarts, Mergetron reclaims the issue back to the ready queue automatically — no manual relabel needed.

[tanflem]

This run found the issue already fully resolved by open PR #9528 (all CI checks passing, mergeable) — the worker already uses children: { set: [...] } instead of connect, and the query already includes videos whose childIds shrank to empty via children: { some: {} }, matching all three acceptance criteria. No further changes made in this run; leaving the existing lease/labels untouched for the merge step to proceed.