feat(admin): new field and routes for date of last mail sent

scripts/init-db.js

src/admin/admin.controller.ts

src/migrations/scripts/1720079233940-date-mail-maj.ts

src/structures/dto/structure.dto.ts

src/structures/schemas/structure.schema.ts

src/structures/services/structures-export.service.spec.ts

src/structures/services/structures.service.spec.ts

src/structures/services/structures.service.ts

test/mock/data/structure.mock.dto.ts

test/mock/data/structures.mock.data.ts

test/mock/services/structures.mock.service.ts