Type Alias AvailabilityLevel

AvailabilityLevel: "Full" | "Degraded" | "Sparse" | "Unavailable" | "Unknown"

Overall feed availability level.

Level	Condition	Description
Full	≥10/13 ok	Normal operation
Degraded	5–9/13 ok	Reduced data quality expected
Sparse	1–4/13 ok	Minimal data, analysis-only mode likely
Unavailable	0/13 ok AND ≥1 probed feed errored	All probed feeds failing
Unknown	0/13 ok AND 0 errors (no probes yet)	Cache is empty — status cannot be determined without a live probe

Unknown is distinct from Unavailable to prevent consumers from interpreting "no cached data" as "feeds are down". See issue #1.