Concurrent feed fetching with durable fetch-result tracking

Feeds are fetched with bounded concurrency (semaphore of 8) instead of
strictly sequentially -- with 100+ subscriptions, one slow/stalling
server no longer delays every feed queued behind it. A process-local
mutex prevents the periodic and manual/alarm-triggered one-time
fetches (two different WorkManager unique-work names, so its own dedup
doesn't cover this) from ever running concurrently and racing writes
to the same feed rows.

Settings.recordFetchResult persists lastFetchAt/lastFetchNewArticles
on every run, not derived from WorkInfo.State -- a PeriodicWorkRequest
cycles back to ENQUEUED after every successful run and never reaches
SUCCEEDED, so anything watching for that only ever saw manual fetches.
ArticleListScreen surfaces this as a "N new articles" snackbar
(acknowledged durably, so returning to the screen doesn't re-show it)
and computes live "Checking feed X of Y" progress from the worker's
tagged WorkInfo, scoped to the Unread/Older views that actually
aggregate every subscription.

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