A feed's failure is only worth saying once it has lasted a day

Peter opened the app to six shows in red, every one of them "Could not reach that feed". All six
answered 200 when checked from the server a minute later.

His guess was an auto-refresh on focus. There isn't one -- the only triggers are the button and a
six-hourly worker -- and that makes it worse rather than better: what he was reading was a stale
report of a connection that dropped during a background refresh hours earlier. The line survives
until that feed next succeeds, and the worker is constrained to unmetered networks, so on mobile
data "next" can be a long time.

The error is still worth storing. A feed that has quietly stopped working is indistinguishable
from one with nothing new, which is the whole reason it is kept rather than logged. But one miss
is noise and a pattern is information, and the difference was already recorded: lastFetchedAtMs
is the last time the feed actually worked. So a failure appears once a feed has gone a day
without succeeding -- about four missed attempts at the current cadence. A dropped connection
clears itself and is never seen; something genuinely broken still goes red, the same day.

No schema change; the timestamp was already there.

Two cases stay loud on purpose. A feed that has *never* succeeded reports immediately, because a
subscription that failed the moment it was added is not a blip and somebody is waiting on it. And
a success timestamp in the future is treated as no success, so a clock that has moved cannot hide
a real failure indefinitely.

Not changed, and worth deciding separately: the periodic refresh is unmetered-only, so on mobile
data the library goes stale with nothing saying so. Adding a refresh on launch is now safe in a
way it was not before -- the transient failures it would produce are exactly what this hides --
but it is a behaviour change nobody asked for.

1052 tests, detekt 0, lint clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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