Cleanup: infer a missing sex from a known-sex spouse (preview → approve)

Unset sex renders blue (male-colored), which is misleading next to a confirmed
male partner. Add a Cleanup action that proposes the opposite sex for an unset
partner of someone whose sex is set (couples are opposite-sex in practice — a
confirmed-male husband ⇒ a female wife). People whose known partners disagree
are skipped as ambiguous.

It's a preview the user reviews and approves in the Cleanup tool (reusing the
existing gender apply path + audit) — not an autonomous write. Backend:
guess_gender_by_spouse + GET /cleanup/gender/from-spouse. Frontend: an "Infer
from spouse" button feeding the existing proposal list. Test covers
propose-opposite, skip-no-partner, skip-already-set, apply, and re-preview.

Full suite 73 passed; frontend build clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/api/v1/cleanup.py

@@ -67,6 +67,16 @@ async def guess_gender(
     return [GenderProposal(**r) for r in rows]
 
 
+@router.get("/{tree_id}/cleanup/gender/from-spouse", response_model=list[GenderProposal])
+async def guess_gender_from_spouse(
+    tree_id: uuid.UUID, session: SessionDep, current: CurrentUser
+) -> list[GenderProposal]:
+    """Infer a missing sex from a partner whose sex is set (opposite-sex couple)."""
+    tree = await tree_service.get_tree(session, viewer_id=current.id, tree_id=tree_id)
+    rows = await cleanup_service.guess_gender_by_spouse(session, actor=current, tree=tree)
+    return [GenderProposal(**r) for r in rows]
+
+
 @router.post("/{tree_id}/cleanup/gender", response_model=CleanupResult)
 async def apply_gender(
     tree_id: uuid.UUID, data: GenderApply, session: SessionDep, current: CurrentUser