Rename host library to master throughout

The two libraries are now "master" and "slave" everywhere: UI text, API
JSON fields (master_root, rel_master, master_size, ...), Go identifiers,
database columns, quality verdicts, quarantine subfolders and docs.

Existing databases are upgraded by a new migration that renames the
columns in place and rewrites stored "host" values, so pairs, history,
cached verdicts and restorable quarantine entries carry over. Network
"host" usages (SplitHostPort, the Origin/Host check) are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/probe/probe.go

@@ -172,48 +172,48 @@ func Score(in Info) float64 {
 	return float64(in.Bitrate) / 1000 * codecEfficiency(in.Codec)
 }
 
-// Verdict compares host and slave quality.
+// Verdict compares master and slave quality.
 type Verdict struct {
-	// Better is "host", "slave", "equal" or "unknown".
+	// Better is "master", "slave", "equal" or "unknown".
 	Better string `json:"better"`
-	// DurationDelta is |host-slave| in seconds; large deltas suggest a
+	// DurationDelta is |master-slave| in seconds; large deltas suggest a
 	// different edit or version of the song rather than a re-encode.
 	DurationDelta float64  `json:"duration_delta"`
 	Notes         []string `json:"notes"`
 }
 
-// Compare ranks host against slave. Within 5% is treated as equal.
-func Compare(host, slave Info) Verdict {
+// Compare ranks master against slave. Within 5% is treated as equal.
+func Compare(master, slave Info) Verdict {
 	v := Verdict{Better: "unknown"}
-	if host.Error != "" || slave.Error != "" {
+	if master.Error != "" || slave.Error != "" {
 		v.Notes = append(v.Notes, "could not probe one of the files")
 		return v
 	}
-	if host.Duration > 0 && slave.Duration > 0 {
-		v.DurationDelta = math.Abs(host.Duration - slave.Duration)
+	if master.Duration > 0 && slave.Duration > 0 {
+		v.DurationDelta = math.Abs(master.Duration - slave.Duration)
 		if v.DurationDelta > 2 {
 			v.Notes = append(v.Notes, fmt.Sprintf("durations differ by %.1fs: possibly a different version or edit", v.DurationDelta))
 		}
 	}
-	hs, ss := Score(host), Score(slave)
+	ms, ss := Score(master), Score(slave)
 	switch {
-	case hs <= 0 || ss <= 0:
-	case math.Abs(hs-ss) <= 0.05*math.Max(hs, ss):
+	case ms <= 0 || ss <= 0:
+	case math.Abs(ms-ss) <= 0.05*math.Max(ms, ss):
 		v.Better = "equal"
-	case hs > ss:
-		v.Better = "host"
+	case ms > ss:
+		v.Better = "master"
 	default:
 		v.Better = "slave"
 	}
-	if host.Lossless != slave.Lossless {
-		if host.Lossless {
-			v.Notes = append(v.Notes, "host is lossless, slave is lossy")
+	if master.Lossless != slave.Lossless {
+		if master.Lossless {
+			v.Notes = append(v.Notes, "master is lossless, slave is lossy")
 		} else {
-			v.Notes = append(v.Notes, "slave is lossless but host is lossy: linking would downgrade the slave")
+			v.Notes = append(v.Notes, "slave is lossless but master is lossy: linking would downgrade the slave")
 		}
 	}
-	if host.Channels != slave.Channels && host.Channels > 0 && slave.Channels > 0 {
-		v.Notes = append(v.Notes, fmt.Sprintf("channel count differs (%d vs %d)", host.Channels, slave.Channels))
+	if master.Channels != slave.Channels && master.Channels > 0 && slave.Channels > 0 {
+		v.Notes = append(v.Notes, fmt.Sprintf("channel count differs (%d vs %d)", master.Channels, slave.Channels))
 	}
 	return v
 }