feat(feed): bound feed lifecycle to 30-day window with nonblocking refresh

Persisted feeds keep only episodes from the last 30 days (plus completed
downloads); older episodes live in volatile memory and survive refreshes
via union merge, with per-feed in-memory caches capped at 500. Refresh
batches run at FETCH_CONCURRENCY=4 with per-feed incremental apply (no
Promise.all barrier), config.json writes are trailing-edge debounced
(250ms, immediate flushPendingSave for unsubscribes), and cold
fetch-more refetches abort at FETCH_TIMEOUT_MS. A shared activity store
powers a global top-right indicator covering refresh, fetch-more,
subscribe, search, and downloads.

Also includes the in-flight incremental RSS parsing (chunked with
event-loop yields) and refresh spinner work this tree already carried.

tasks/bounded-feed-lifecycle/README.md

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+# Bounded Feed Lifecycle
+
+Objective: Bound feed episode storage to a rolling 30-day persisted window (older episodes volatile-only unless downloaded), keep feed loading nonblocking, and surface all load/download activity in a global top-right indicator.
+
+Status legend: [ ] todo, [~] in-progress, [x] done
+
+Tasks
+
+- [x] 01 — persisted-retention-window → `01-persisted-retention-window.md`
+- [x] 02 — volatile-episode-merge → `02-volatile-episode-merge.md`
+- [x] 03 — nonblocking-feed-refresh → `03-nonblocking-feed-refresh.md`
+- [x] 04 — global-activity-indicator → `04-global-activity-indicator.md`
+
+Dependencies
+
+- 02 depends on 01 (the volatile merge preserves exactly what 01 drops from disk)
+- 03 depends on 01 (debounced persistence layers onto the pruning save path)
+- 03 depends on 02 (incremental per-feed apply consumes the merge helper from 02)
+- 04 depends on 03 (the indicator subscribes to the activity wiring added across refresh/load-more paths in 03)
+
+Exit criteria
+
+- After any refresh + save, `config.json` `feeds[*].episodes` contains only episodes with `pubDate` within the last 30 days or episodes marked `completed` in `downloads.json`; loading a legacy config prunes stale episodes on first launch.
+- In-memory retention is capped per feed; episodes aged out of the persisted window remain browsable within the session and are re-fetchable via fetch-more after a restart.
+- A refresh batch never exceeds a fixed fetch concurrency, applies each feed's result as it lands (no `Promise.all` barrier), and persistence writes are debounced; keyboard input stays responsive throughout.
+- The top-right indicator is visible iff at least one feed refresh, fetch-more, subscribe fetch, search, or episode download is in flight, hidden otherwise.
+- `bun test` and `bun run lint` pass.