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>

frontend/lib/api/schema.d.ts

@@ -718,6 +718,27 @@ export interface paths {
         patch?: never;
         trace?: never;
     };
+    "/api/v1/trees/{tree_id}/cleanup/deceased-by-child": {
+        parameters: {
+            query?: never;
+            header?: never;
+            path?: never;
+            cookie?: never;
+        };
+        /**
+         * Preview Deceased By Child
+         * @description People with a child born on/before the cutoff — necessarily deceased even
+         *     when their own birth date is missing. Apply via POST .../cleanup/deceased.
+         */
+        get: operations["preview_deceased_by_child_api_v1_trees__tree_id__cleanup_deceased_by_child_get"];
+        put?: never;
+        post?: never;
+        delete?: never;
+        options?: never;
+        head?: never;
+        patch?: never;
+        trace?: never;
+    };
     "/api/v1/trees/{tree_id}/cleanup/gender/preview": {
         parameters: {
             query?: never;
@@ -1286,6 +1307,18 @@ export interface components {
             /** Person Ids */
             person_ids: string[];
         };
+        /** DeceasedByChildCandidate */
+        DeceasedByChildCandidate: {
+            /**
+             * Person Id
+             * Format: uuid
+             */
+            person_id: string;
+            /** Name */
+            name: string;
+            /** Child Birth Year */
+            child_birth_year: number;
+        };
         /** DeceasedCandidate */
         DeceasedCandidate: {
             /**
@@ -4013,6 +4046,39 @@ export interface operations {
             };
         };
     };
+    preview_deceased_by_child_api_v1_trees__tree_id__cleanup_deceased_by_child_get: {
+        parameters: {
+            query?: {
+                born_on_or_before?: number;
+            };
+            header?: never;
+            path: {
+                tree_id: string;
+            };
+            cookie?: never;
+        };
+        requestBody?: never;
+        responses: {
+            /** @description Successful Response */
+            200: {
+                headers: {
+                    [name: string]: unknown;
+                };
+                content: {
+                    "application/json": components["schemas"]["DeceasedByChildCandidate"][];
+                };
+            };
+            /** @description Validation Error */
+            422: {
+                headers: {
+                    [name: string]: unknown;
+                };
+                content: {
+                    "application/json": components["schemas"]["HTTPValidationError"];
+                };
+            };
+        };
+    };
     preview_gender_api_v1_trees__tree_id__cleanup_gender_preview_post: {
         parameters: {
             query?: never;