Rename `cited_by_count_date` to `cited_by_count_year`

[ssheikholeslami]

cited_by_count_date now stores a year integer rather than a full date/time, so the field name is misleading and can cause confusion for future readers/DB consumers. Consider renaming it to something like cited_by_count_year (and, if you need to keep the persisted property name stable, use a Pydantic alias or a clear field description).

    cited_by_count_year: Optional[int] = Field(
        default=None,
        alias="cited_by_count_date",
        description="Year associated with cited_by_count; persisted as 'cited_by_count_date'.",
    )

Originally posted by @Copilot in #42 (comment)