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_test.go

@@ -52,6 +52,6 @@ func TestProbeRealFiles(t *testing.T) {
 		t.Fatalf("unexpected probe results %+v / %+v", fi, mi)
 	}
 	if v := Compare(mi, fi); v.Better != "slave" {
-		t.Fatalf("flac slave should beat mp3 host, got %+v", v)
+		t.Fatalf("flac slave should beat mp3 master, got %+v", v)
 	}
 }