Fix: delete stale Download objects when clearing cache

[GeiserX]

Summary

• Fixes [BUG] "Download all songs in library" silently does nothing after switching download format or clearing cache #662 — "Download all songs in library" silently does nothing after clearing cache or switching download format

Root Cause

Two issues combine to produce the bug:

1. Stale DownloadMO after cache clear (preventive fix — commit 1)

deleteCacheFinalStep (called by "Delete downloaded songs and podcast episodes") sets relFilePath = nil but leaves the associated DownloadMO Core Data relationship intact. This follows the same pattern already used when resolving duplicate songs (line 185-186) and podcast episodes (line 217-218), which already delete the associated DownloadMO via context.delete(download).

2. Overly restrictive fetch predicate (self-healing fix — commit 2)

getSongsForCompleteLibraryDownload requires both predicates to match:

NSPredicate(format: "%K == nil", #keyPath(SongMO.relFilePath)),  // ✅ cleared by cache delete
NSPredicate(format: "%K == nil", #keyPath(SongMO.download)),      // ❌ stale DownloadMO still present

The download == nil predicate is redundant with DownloadRequestManager.addLowPrio, which already correctly handles existing downloads: if the song is not cached (!object.isCached), it resets the download and re-queues it. Songs that are already cached are skipped. Removing this predicate allows addLowPrio's own deduplication logic to handle both fresh and stale state.

Why both commits

• Commit 1 (preventive): Deletes DownloadMO during cache clear so the stale state is never created going forward.
• Commit 2 (migratory): Removes the redundant download == nil predicate so users upgrading from older builds self-heal on next "Download all" without manual cleanup.

Test Plan

• Fresh account: sync library → "Download all songs in library" → downloads start normally
• Cache clear cycle: let some downloads complete → "Delete downloaded songs and podcast episodes" → "Download all songs in library" → second run queues songs successfully
• Already-broken account from older build: upgrade to patched build → "Download all songs in library" → stale state self-heals and downloads resume
• Normal download flow (without cache clear) still works as before
• Songs currently being downloaded are not disrupted by a concurrent "Download all"

[GeiserX]

Validation Results

Tested both commits on macOS (patched build from this PR branch) against Navidrome with ALAC library (~1636 songs).

Repro matrix

#	Account URL	Cache state before test	Action	Expected	Result
1	192.168.99.99:4534 (LAN IP)	Fresh account, no stale DownloadMOs	"Download all songs"	Downloads start	PASS (worked even before fix — clean bucket)
2	my.tail-scale.ts.net:4534 (hostname)	1655 stale DownloadMOs (download exists, relFilePath == nil)	"Download all songs"	Downloads start (self-healing)	PASS — all 1655 songs re-queued and streaming
3	Either account	After cache clear + re-download	"Download all songs"	Downloads start	PASS — commit 1 prevents stale state from forming

Key observations

• Account 1 (hostname) had 1655 DownloadMO objects with no associated cached file — classic stale state from the old deleteCacheFinalStep not cleaning up downloads.
• Before the fix, "Download all songs" returned 0 candidates because getSongsForCompleteLibraryDownload required download == nil.
• After the fix (commit 2), addLowPrio correctly detected !object.isCached on the existing stale downloads, reset them, and re-queued — confirmed by Navidrome streaming logs showing all songs downloading in sequence.
• Commit 1 ensures this stale state won't recur for future cache clears.

Navidrome log excerpt (hostname account, post-fix)

Streaming file artist=U3 format=raw originalFormat=m4a title=""40""
Streaming file artist="The Rolling Mones" format=raw originalFormat=m4a title="(I Can't Get No) Satisfaction"
Streaming file artist="David Mowie" format=raw originalFormat=m4a title="\"Heroes\""
Streaming file artist="Blue Pinkster Cult" format=raw originalFormat=m4a title="(Don't Fear) The Reaper"
Streaming file artist=Pandiohead format=raw originalFormat=m4a title="15 Step"
... (1655 songs total)