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/download/provider.go

@@ -130,6 +130,12 @@ type Config struct {
 	Enabled  bool              `json:"enabled"`
 	Priority int               `json:"priority"`
 	Settings map[string]string `json:"settings"`
+
+	// SetSecrets names which of the descriptor's secret fields already
+	// have a stored value, without exposing it.  Populated only when a
+	// Config is built for the frontend (see Service.withSecretFlags);
+	// empty when read from or written to the store.
+	SetSecrets map[string]bool `json:"setSecrets,omitempty"`
 }
 
 // Setting returns a config value, or fallback when unset.
@@ -197,6 +203,11 @@ type Field struct {
 	// provider config row, and rendered as a password input.
 	Secret bool `json:"secret"`
 
+	// Path marks a value that names a local filesystem directory, so
+	// the settings form can offer a native folder picker beside the
+	// text input rather than making the user type or paste it.
+	Path bool `json:"path"`
+
 	Required bool   `json:"required"`
 	Default  string `json:"default,omitempty"`
 }