availableLanguages: seedVideoLanguages stops zeroing out container videos

[tanflem]

Parent

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

What to build

Running the seedVideoLanguages worker against a container video (collection/series/featureFilm) no longer zeroes out its child-derived availableLanguages. Today it overwrites the field from the video's own variants only, ignoring children entirely — a container with no direct variants of its own ends up with [] even if its children have languages. It's changed to route through the recompute engine from #9517.

Acceptance criteria

• Seeding a container video with no variants of its own but with children that have published-variant languages results in the container's availableLanguages correctly reflecting those child-derived languages, not [].
• Seeding a leaf video (no children) continues to work as before — its own variant languages are set correctly.
• Existing seedVideoLanguages tests are updated to assert against the new (correct) behavior rather than the own-variants-only behavior.

Blocked by

• availableLanguages: cycle-safe, cascade-to-fixpoint recompute engine #9517 (needs the recompute engine as the write path)

[tanflem]

🔒 Mergetron holds a lease on this issue — runtime 23d237d8-ffd4-4f4e-960e-7682f69cf890, branch phoebe/bot-0-issue-9520.

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]

Addressed by Phoebe: #9533

[tanflem]

🔒 Mergetron holds a lease on this issue — runtime 23d237d8-ffd4-4f4e-960e-7682f69cf890, branch phoebe/bot-0-issue-9520.

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.