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/schemas/cleanup.py

@@ -9,6 +9,12 @@ class DeceasedCandidate(BaseModel):
     birth_year: int
 
 
+class DeceasedByChildCandidate(BaseModel):
+    person_id: uuid.UUID
+    name: str
+    child_birth_year: int
+
+
 class DeceasedApply(BaseModel):
     person_ids: list[uuid.UUID]