Backstop for the Farcaster vault-visibility gap.

handle_cast writes the vault-visible spine row (onchain_publish_events, keyed farcaster_<content_id>) best-effort with no backstop — a failed write leaves a source="farcaster_auto" cast owned/earning but INVISIBLE in the creator's vault (the vault filters onchain_publish_events by contributors_jubjub_profile_ids). This sweep finds such orphans and (re)writes the spine row via record_launch_onchain + _LaunchProxy, reconstructing the launch from the content doc's own fields. Idempotent + safe:

• The staged doc is written with .set(merge=True) keyed by launch_id, so a double write merges harmlessly.
• We SKIP any row that already has contributors_jubjub_profile_ids. That field is only set once record_launch_onchain reaches step 5 (the full staged write), which is BEFORE its on-chain ledger tx (step 6). So "no contributors" ⟹ the on-chain tx never fired ⟹ running record_launch_onchain fires it exactly once (no double-submit); and any already-staged/confirmed row (contributors present) is left untouched.

NOTE: the generic spine-recovery worker (recover_stuck_spine_events) CANNOT heal these — it loads the launch from launches_v2, which Farcaster casts do not have. Hence this dedicated sweep.

Called by Cloud Scheduler (needs a NEW job — see TODO). Best-effort, batched.