fix(indexexport): read an index older than the binary

`maintain-index` failed with

    indexexport: copy rows: SQL logic error: no such column: total_tracks

three minutes into the one job that owns the ~205 GB checkpoint and
publishes the catalog every user downloads.

The cause is the exception that keeps that checkpoint alive. The job's
/cache is a real YJ_HOME that survives between runs, so its
explore_index is classified Cache and is deliberately *not* dropped and
recreated by cmd/indexbuild's schema repair -- which means a column
added to the schema afterwards is absent from it. total_tracks arrived
with the album-completeness work; the exporter selected it regardless.

The fix is the rule the importing side already follows.
artifactHasTotals exists because "adding a column to the importer's
SELECT is how you break every artifact already published"; the mirror
image, reading an index older than the binary, had no such guard.
sourceColumns asks pragma_table_info and selects a literal 0 when the
column is absent -- which is what that column already means by "the
catalog does not say", and what the app renders as unknown rather than
as incomplete. The artifact keeps every column, so an importer needs no
second shape.

The test reproduces the failure symptom first: with the fix removed it
fails with the CI message verbatim. Its own first version proved
nothing, though, and that is worth the comment it now carries --
`strings.Replace(catalogColumns, "total_tracks, ", …)` matches nothing,
because the list is formatted across lines and the name is followed by
a newline, so the "old" index was built with every current column.

cmd/indexexport/main.go

@@ -25,6 +25,7 @@ import (
 	"os"
 	"path/filepath"
 	"strconv"
+	"strings"
 	"time"
 
 	_ "modernc.org/sqlite"
@@ -43,6 +44,41 @@ const catalogColumns = `entity_type, mbid, title, artist_name, artist_mbid,
 	release_name, primary_type, secondary_types, release_date, total_tracks,
 	artist_type, country, disambiguation, sort_name, discog_fetched`
 
+// sourceColumns is catalogColumns as read *from* the built index,
+// which is not always shaped like the one this binary was compiled
+// against.
+//
+// The index job's /cache volume is a real YJ_HOME that survives
+// between runs and holds ~205 GB nobody can re-download casually, so
+// its explore_index is classified Cache and is deliberately **not**
+// dropped and recreated by cmd/indexbuild's schema repair. A column
+// added to the schema after that database was built is therefore
+// absent from it, and selecting it fails the whole export with
+// "no such column: total_tracks" -- which is what happened the first
+// time the job ran after the completeness work.
+//
+// So the source list is asked for rather than assumed, exactly as
+// artifactHasTotals does on the importing side. Zero is what the
+// column means by "the catalog does not say", and the app already
+// renders that as unknown rather than as incomplete.
+func sourceColumns(db *sql.DB) string {
+	var n int
+
+	err := db.QueryRow(
+		`SELECT COUNT(*) FROM pragma_table_info('explore_index', 'main')
+		 WHERE name = 'total_tracks'`,
+	).Scan(&n)
+	if err == nil && n > 0 {
+		return catalogColumns
+	}
+
+	fmt.Println(
+		"  note: this index predates total_tracks; exporting 0 for it",
+	)
+
+	return strings.Replace(catalogColumns, "total_tracks", "0", 1)
+}
+
 var errNoHome = errors.New(
 	"YJ_HOME must be set to the directory holding the built index",
 )
@@ -189,6 +225,10 @@ func createSchema(db *sql.DB) error {
 // dumpcatalog.go — a flat global top-N would give a handful of
 // superstars everything and everyone else nothing.
 func copyRows(db *sql.DB, artists, perArtistRGs, perArtistRecs int) error {
+	// The destination is created by this binary and always has every
+	// column; only the source may be older.
+	srcColumns := sourceColumns(db)
+
 	if _, err := db.Exec(`
 		CREATE TEMP TABLE core_artists AS
 		SELECT mbid FROM main.explore_index
@@ -201,7 +241,7 @@ func copyRows(db *sql.DB, artists, perArtistRGs, perArtistRecs int) error {
 
 	copied, err := insertSelect(db, `
 		INSERT INTO core.explore_index (`+catalogColumns+`)
-		SELECT `+catalogColumns+`
+		SELECT `+srcColumns+`
 		FROM main.explore_index
 		WHERE entity_type = 1 /* artist */
 		  AND mbid IN (SELECT mbid FROM core_artists)`)
@@ -228,7 +268,7 @@ func copyRows(db *sql.DB, artists, perArtistRGs, perArtistRecs int) error {
 		// most `limit` rows, ranked by their own listen counts.
 		n, err := insertSelect(db, `
 			INSERT INTO core.explore_index (`+catalogColumns+`)
-			SELECT `+catalogColumns+` FROM (
+			SELECT `+srcColumns+` FROM (
 				SELECT *, ROW_NUMBER() OVER (
 					PARTITION BY artist_mbid ORDER BY popularity DESC
 				) AS rn