fix(download): search Soulseek more than once, and read file lengths

The slskd search asked one question and ignored part of the answer.

Two queries.  Soulseek matches every term against a file's full path,
so every extra word is a filter, and several filter wrongly: an edition
qualifier from the catalog title that no one puts in a folder name, a
term with a leading "-", which Soulseek reads as an exclusion, and
"Various Artists", which is in no one's path.  When a normalised form of
the request differs, it runs alongside the original and the candidates
are merged by peer and folder.  Concurrently, not as a fallback: the
manager gives a provider one search budget, and a Soulseek search spends
most of it waiting.  A query the user typed is searched as written.

Stated options.  The search carried only its id and text, so slskd's
own defaults for its timeout and response limits applied.  Its timeout
is now set inside our wait, the limits are well above a popular album,
and slskd drops folders below the file floor and peers with a queue we
would not reach today.

A state-only poll.  Every one-second poll re-sent every response; the
responses are now fetched once at the end, falling back to the old
includeResponses form for a daemon without that endpoint.

Durations.  slskd reports each file's length and it was discarded.  It
is now carried as CandidateFile.LengthMillis and scored against the
expected tracks as DurationFit, which takes 0.15 of title fit's weight
when at least half the aligned pairs are timed: a title says which song
a file claims to be, a length says whether it is that recording.
Without lengths the score is exactly the previous formula.

freeUploadSlots is removed from the response type; slskd sends
hasFreeUploadSlot and nothing by that name.

Closes #271

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017HJiuc3ZZhxsPXz3ozTirT

backend/download/rank.go

@@ -35,6 +35,15 @@ const (
 	weightArtistFit    = 0.12
 )
 
+// Match sub-weights when the candidate's durations are known.  Duration
+// takes its weight from title fit, the signal it corroborates: a title
+// says which song a file claims to be, a length says whether it is that
+// recording — the right edit, the whole file, not the live take.
+const (
+	timedWeightTitleFit    = 0.25
+	timedWeightDurationFit = 0.15
+)
+
 // Quality sub-weights.  Each set sums to 1.0.
 //
 // There are two of them because a stated preference changes what the
@@ -319,13 +328,13 @@ func Score(dl Download, c Candidate, priority int, prefs AutoDownloadPrefs) Cand
 
 	audio := c.AudioFiles()
 
-	matched, titleFit := matchFiles(audio, dl.Expected)
+	a := alignFiles(audio, dl.Expected)
 
 	// Write the alignment back so the picker can show which file maps
 	// to which track.
-	c.Files = mergeMatched(c.Files, matched)
+	c.Files = mergeMatched(c.Files, a.files)
 
-	c.Match = scoreMatch(dl, c, audio, titleFit)
+	c.Match = scoreMatch(dl, c, audio, a)
 	c.Quality = scoreQuality(
 		c, audio, priority, prefs, dl.runtimeMillis(),
 	)
@@ -340,11 +349,16 @@ func scoreMatch(
 	dl Download,
 	c Candidate,
 	audio []CandidateFile,
-	titleFit float64,
+	a alignment,
 ) MatchScore {
 	m := MatchScore{
-		Anchored: dl.Anchored(),
-		TitleFit: titleFit,
+		Anchored:    dl.Anchored(),
+		TitleFit:    a.titleFit,
+		DurationFit: a.durationFit,
+
+		// Durations count once at least half the aligned pairs state
+		// one; a single timed pair would be a coin toss carrying 15%.
+		DurationKnown: a.timedPairs > 0 && a.timedPairs*2 >= a.aligned,
 	}
 
 	m.Completeness = completeness(
@@ -369,9 +383,16 @@ func scoreMatch(
 	// With no expected tracklist there is no title signal at all, so
 	// redistribute its weight onto the album/artist evidence rather
 	// than scoring every free-text result as half-wrong.
-	if len(dl.Expected) == 0 {
+	switch {
+	case len(dl.Expected) == 0:
 		m.Overall = 0.55*m.AlbumFit + 0.45*m.ArtistFit
-	} else {
+	case m.DurationKnown:
+		m.Overall = timedWeightTitleFit*m.TitleFit +
+			timedWeightDurationFit*m.DurationFit +
+			weightCompleteness*m.Completeness +
+			weightAlbumFit*m.AlbumFit +
+			weightArtistFit*m.ArtistFit
+	default:
 		m.Overall = weightTitleFit*m.TitleFit +
 			weightCompleteness*m.Completeness +
 			weightAlbumFit*m.AlbumFit +