Finalise a claim: freeze its answers, lock further edits, and notify.

Checks, in order:

1. Current status allows transition to submitted per the claim type's transition table.
2. No unresolved root review comments.
3. Company is not a subsidiary of a consolidated/MEC group (eligibility rule for AU R&D Tax Incentive).
4. No unanswered required questions.

On success the handler sets status, submitted_at, submitted_by_id, and freezes the current questions payload into submitted_snapshot. Writes a ClaimAuditLog entry and dispatches a claim.submitted notification. Discord failures are swallowed — the submission commits regardless.

Scope of the v1 edit lock: get_editable_claim_for_user blocks answer writes, comment writes, and renames on a submitted claim. It does not block uploads in api/files.py or AURD tracking mutations in api/aurd_tracking.py — those endpoints use different auth paths. The submitted_snapshot is therefore the record of which answers were filed, not of supporting material state at submission. See CLA-127 follow-ups.