Replace the hand-rolled worker pool (mapWithConcurrency) with an Effect
program (src/effects/feed-refresh.ts): Effect.forEach bounds in-flight
fetches, Effect.timeout bounds each feed via the Clock service, and
failures fold to null so a bad feed never fails the batch. Per-feed
apply-as-it-lands is preserved — the apply callback runs inside each
feed's own fiber, so there is no Promise.all barrier.
Store boundary unchanged: refreshAllFeeds runs the program through
Effect.runPromise, keeping runAutoDownload + flushPendingSave after the
batch and isLoadingFeeds around it.
Adds TestClock-driven tests (tests/feed-refresh-effect.test.ts) that pin
concurrency, per-feed apply, timeout, and failure isolation without real
20s waits. Pins effect@^3 (V4 is in beta).
