Add timestamp column to RadialVelocities table

Radial velocities change with time, especially for sources with orbiting companions. A timestamp for RVs is pretty important. I am not sure what to call the column, however. But I think it should be in the database as a timestamp string in ISO format and parseable into a Python DateTime object.

The payload names no files, tests, or entry points. Locate the schema definition for the RadialVelocities table and inspect how existing date fields are represented; resolve the column name and timestamp requirements with maintainers before implementing, with completion confirmed by the project’s schema validation or database tests.

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