feat: autotag mixed-bag splitting, search relevance fixes, and multi-library download imports

Autotag: detect "junk drawer" folders with no artist/album consensus
and split them into synthetic per-cluster groups instead of forcing
one match on an unrelated pile of tracks; repair tagging_items rows
left behind by a prior scan orphan-cleanup gap.

Explore: fix an exact artist-name search being drowned out by its own
catalog entries in intent-prior scoring, and prune stale in_library
bookkeeping left behind when a referenced library row is deleted.

Download: fix a multi-library regression where every import failed
with "no library root configured" — the importer resolved the
library root from a legacy single-library config field that nothing
populates in the current multi-library model. It now resolves the
destination library per-request from the request's own library_id.
Also widen the Soulseek search window (12s -> 20s), measured against
real request history to be missing available peers on live queries.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2Agd9af5hE7qzti2ackiS

backend/autotag/groupkey.go

@@ -19,12 +19,14 @@ import (
 //
 //	libraryID || 0 || normalized_parent_dir || 0 || disc_number
 //
-// where the parent directory is lower-cased.  The folder is taken
-// as the album boundary — including the album tag string would
-// fragment albums whose tracks carry slightly different tags
-// (`Abbey Road` vs `Abbey Road (Remastered 2009)`, etc.).  The
-// album name is still surfaced in `tagging_items.album_name` for
-// the review UI; it just doesn't decide grouping.
+// where the parent directory is lower-cased and disc_number is
+// normalized so an untagged disc (0) folds into disc 1 — see
+// normalizeDiscNumber.  The folder is taken as the album boundary —
+// including the album tag string would fragment albums whose tracks
+// carry slightly different tags (`Abbey Road` vs `Abbey Road
+// (Remastered 2009)`, etc.).  The album name is still surfaced in
+// `tagging_items.album_name` for the review UI; it just doesn't
+// decide grouping.
 //
 // Using SHA-1 matches the codebase's existing non-crypto
 // deterministic-key convention; collision risk at album-group
@@ -41,7 +43,60 @@ func GroupKey(
 	h.Write([]byte{0})
 	h.Write([]byte(parentDir))
 	h.Write([]byte{0})
-	h.Write([]byte(strconv.Itoa(discNumber)))
+	h.Write([]byte(strconv.Itoa(normalizeDiscNumber(discNumber))))
 
 	return hex.EncodeToString(h.Sum(nil))
 }
+
+// SyntheticGroupKey returns a deterministic identifier for a
+// tag-clustered sub-group carved out of parentGroupKey by
+// SplitMixedFolder — same SHA-1-over-null-separated-fields shape as
+// GroupKey, but keyed on the cluster's (album, album-artist) tags
+// instead of a directory, since a synthetic group's tracks don't
+// share a directory boundary distinct from their siblings left
+// behind in the parent folder.
+func SyntheticGroupKey(parentGroupKey, albumName, albumArtist string) string {
+	h := sha1.New() //nolint:gosec // see package doc — grouping only.
+	h.Write([]byte(parentGroupKey))
+	h.Write([]byte{0})
+	h.Write([]byte(Normalize(albumName)))
+	h.Write([]byte{0})
+	h.Write([]byte(Normalize(albumArtist)))
+
+	return hex.EncodeToString(h.Sum(nil))
+}
+
+// SyntheticTrackGroupKey returns a deterministic identifier for a
+// single leftover track carved out of a mixed-bag folder by
+// SplitMixedFolder's singleton fallback (autotag.SplitPlan). Keyed on
+// the track's own audio_files id rather than its tags — two
+// untagged leftover tracks would otherwise both normalize to the
+// same empty (album, album-artist) pair and collide under
+// SyntheticGroupKey.
+func SyntheticTrackGroupKey(parentGroupKey string, audioFileID int64) string {
+	h := sha1.New() //nolint:gosec // see package doc — grouping only.
+	h.Write([]byte(parentGroupKey))
+	h.Write([]byte{0})
+	h.Write([]byte("track"))
+	h.Write([]byte{0})
+	h.Write([]byte(strconv.FormatInt(audioFileID, 10)))
+
+	return hex.EncodeToString(h.Sum(nil))
+}
+
+// normalizeDiscNumber folds a missing/invalid disc tag (<= 0) into
+// disc 1 for grouping purposes.  Without this, a folder where only
+// some tracks carry an explicit "disc 1 of 1" tag — common when
+// files were ripped or re-tagged at different times — splits into
+// two tagging groups for what is really one single-disc album: the
+// untagged tracks hash to disc 0, the tagged ones to disc 1.  A
+// genuine multi-disc release still separates correctly, since its
+// disc-2-and-up tracks carry an explicit non-zero, non-one disc
+// number.
+func normalizeDiscNumber(discNumber int) int {
+	if discNumber <= 0 {
+		return 1
+	}
+
+	return discNumber
+}