Group label not re-derived when validating after members are classified

Problem

Sequence-group label propagation only fires in one direction: propagate_group_label (in annotation_api/src/app/api/api_v1/endpoints/sequence_annotations.py) runs when a member annotation is saved to SEQ_ANNOTATION_DONE, and bails silently if the group is not yet validated:

group = await session.get(SequenceGroup, seq.sequence_group_id)
if group is None or not group.is_validated:
    return None

Meanwhile PATCH /api/v1/sequence_groups/{id} only flips is_validated — it never re-derives the group label from members that already reached SEQ_ANNOTATION_DONE.

So the classify-then-validate ordering strands the group: the annotator classifies a member sequence first (no propagation — group not validated, no warning either), then validates the group (no re-derivation), and the group sits at "to label + validated" indefinitely until some member annotation happens to be re-saved.

Steps to reproduce

1. Take a group with is_validated = false and no label.
2. Classify one of its member sequences to SEQ_ANNOTATION_DONE.
3. Validate the group on /classify/groups/:id.
4. The group stays unlabeled (smoke_type / false_positive_type both null) with no path forward from the UI.

Proposed fix

When the PATCH flips is_validated to true, re-derive the group label from existing member annotations at SEQ_ANNOTATION_DONE (reusing the derivation + conflict rules already in propagate_group_label, including the refuse-to-clobber-conflicting-labels behavior) and fan it out to unlocked members, same as the annotation-save path.

Related

• PR #252 fixed the UI side only (tooltips on /classify/groups now explain that validation confirms the grouping and that propagation requires a validated group).
• If the backend behavior changes, sweep the related copy — the "label propagates once confirmed" phrasing is shared by the Reviewed column tooltip, the Label column tooltip, the page's info popover, and the group page instructions.

Start in annotation_api/src/app/api/api_v1/endpoints/sequence_annotations.py and trace propagate_group_label, then inspect the PATCH /api/v1/sequence_groups/{id} handler. Reuse the existing derivation, conflict, and unlocked-member behavior when validation changes; done means classify-then-validate derives the group label and the related UI copy remains accurate.