feat(feed): make episode cache bound user-configurable (date window or count)

Add episodeCacheMode/count/days preferences (default: date, 60 days).
Apply the bound when reading instead of writing, so a preference change
takes effect without a refetch; the full parse cache stays intact so
fetch-more can page beyond the bound. Thread the window through
load/saveFeedsToFile and update tests and task docs.

src/utils/episode-merge.ts

@@ -1,4 +1,4 @@
-import type { Episode } from "../types/episode"
+import type { Episode } from "../types/episode";
 
 /** Sort key for an episode's pubDate — missing/invalid dates sort as NEWEST
  *  (Infinity) so undated episodes float to the top instead of dropping into
@@ -10,17 +10,21 @@ const ts = (ep: Episode): number => {
 
 /**
  * Union of two episode lists keyed by id — on collision the fetched copy
- * wins (fresh metadata). Result is sorted newest-first by pubDate and capped
- * at `cap` entries (oldest dropped). Never mutates either input.
+ * wins (fresh metadata). Result is sorted newest-first by pubDate and pruned
+ * by the supplied `keep` predicate: episodes outside the configured cache
+ * bound (date window or count) are dropped. Never mutates either input.
+ *
+ * The caller supplies `keep` so this module stays free of the preference
+ * types — the feed store passes a closure bound to the user's mode/count/days.
  */
-export function mergeEpisodes(
+export function mergeEpisodesBounded(
   existing: Episode[],
   fetched: Episode[],
-  cap: number,
+  keep: (ep: Episode, index: number) => boolean,
 ): Episode[] {
   const byId = new Map<string, Episode>()
   for (const ep of existing) byId.set(ep.id, ep)
   for (const ep of fetched) byId.set(ep.id, ep)
   const sorted = [...byId.values()].sort((a, b) => ts(b) - ts(a))
-  return sorted.slice(0, cap)
+  return sorted.filter((ep, i) => keep(ep, i))
 }