Observation history for one tracked FRED series

Every stored observation of series_id, oldest first.

ONLY THE SERIES KRESMION ITSELF FETCHES are served; anything else is 404. This is a licence boundary, not a performance one: FRED's terms prohibit applications that replicate the essential user experience of the FRED API, so this is a history of the series behind our own surfaces and never an arbitrary-id pass-through.

date is the observation's own date — the period the number describes — never the day we fetched it. as_of is when we last confirmed these rows against FRED, which is a different fact.