feat(pinchflat): Retention & quality-upgrade per Library source

On-demand maintenance panel (SourceRetention) on the source detail —
not a persisted schedule (roadmap's niche-for-desktop scope).

Prune: keep the newest N downloaded files, delete the rest's files
(download archive prevents silent re-download); dry-run count + two-step
confirm before any deletion. Upgrade: re-enqueue items whose recorded
downloadedQuality ranks below the source's current target (profile or
global). Selection is pure + injected-now in @shared/retention.ts
(selectPruneCandidates/selectUpgradeCandidates + qualityRank), unit-
tested; pruneMediaItems (main) deletes best-effort and clears state.
MediaItem.downloadedQuality now recorded through the completion path.
Live-verified the panel renders. 366 tests pass.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ROADMAP-PINCHFLAT.md

@@ -199,8 +199,18 @@ Re-running a channel should grab only what's new — the everyday media-manager
       backed by the existing **`--download-archive`** setting (`Settings.downloadArchive`,
       `getDownloadArchivePath()`) as a second, yt-dlp-level dedup guard so even a stale index
       can't re-download.
-- [ ] **Retention / quality-upgrade (stretch, from Pinchflat).** Optional re-download to upgrade
-      quality/metadata, and optional pruning of old items. Defer — niche for a desktop app.
+- [x] **Retention / quality-upgrade (stretch, from Pinchflat).** Optional re-download to upgrade
+      quality/metadata, and optional pruning of old items. Implemented as an on-demand per-source
+      maintenance panel (`SourceRetention` on the Library source detail), not a persisted schedule
+      (matching the "niche for a desktop app" scope). **Prune:** keep the newest N downloaded files,
+      delete the older ones' files (the download archive stops them silently re-downloading); shows a
+      dry-run count and a two-step confirm before any deletion. **Upgrade:** re-enqueue items whose
+      recorded `downloadedQuality` ranks below the source's current target (its profile's quality or
+      the global default). The selection logic is pure + injected-now (`@shared/retention.ts`:
+      `selectPruneCandidates`/`selectUpgradeCandidates`, `qualityRank` in `@shared/ipc`), unit-tested
+      in `test/retention.test.ts`; the impure prune (`pruneMediaItems` in `src/main/sources.ts`)
+      deletes files best-effort and clears `downloaded`. `MediaItem.downloadedQuality` is now recorded
+      through the completion path. Live-verified the panel renders on an expanded source.
 
 ## Phase J — Subscriptions & scheduled auto-download ✅ COMPLETE (OS wiring needs smoke test)