Cleanup tool: "mark deceased by a child's birth year" rule

Adds a preview/apply rule to the Cleanup tool for parents who have NO birth date
of their own (so the existing born-on-or-before rule can't reach them) but who
have a child born long ago — they're necessarily deceased. This is the gap that
left ~56 parents in the Paul tree as "unknown".

- cleanup_service.preview_deceased_by_child(year): parents of any child born
  on/before the cutoff, excluding already-deceased; returns child_birth_year.
- GET /trees/{id}/cleanup/deceased-by-child?born_on_or_before=1900. Apply reuses
  the existing POST .../cleanup/deceased (same audited mark-deceased path).
- Frontend: a new card in the Cleanup tool (year input → preview → select →
  apply), preview-first like the rest of the tool.

Test covers preview (finds the no-birthdate parent of a pre-cutoff child,
excludes modern-child parents), child_birth_year, apply, and re-preview drop.
Suite 106 passing.

Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/api/v1/cleanup.py

@@ -6,6 +6,7 @@ from app.api.deps import CurrentUser, SessionDep
 from app.schemas.cleanup import (
     CleanupResult,
     DeceasedApply,
+    DeceasedByChildCandidate,
     DeceasedCandidate,
     GenderApply,
     GenderProposal,
@@ -31,6 +32,24 @@ async def preview_deceased(
     return [DeceasedCandidate(**r) for r in rows]
 
 
+@router.get(
+    "/{tree_id}/cleanup/deceased-by-child", response_model=list[DeceasedByChildCandidate]
+)
+async def preview_deceased_by_child(
+    tree_id: uuid.UUID,
+    session: SessionDep,
+    current: CurrentUser,
+    born_on_or_before: int = 1900,
+) -> list[DeceasedByChildCandidate]:
+    """People with a child born on/before the cutoff — necessarily deceased even
+    when their own birth date is missing. Apply via POST .../cleanup/deceased."""
+    tree = await tree_service.get_tree(session, viewer_id=current.id, tree_id=tree_id)
+    rows = await cleanup_service.preview_deceased_by_child(
+        session, actor=current, tree=tree, year=born_on_or_before
+    )
+    return [DeceasedByChildCandidate(**r) for r in rows]
+
+
 @router.post("/{tree_id}/cleanup/deceased", response_model=CleanupResult)
 async def apply_deceased(
     tree_id: uuid.UUID, data: DeceasedApply, session: SessionDep, current: CurrentUser