refactor(scanner): query column nullability with a single-row pragma_table_info select

Replace the manual PRAGMA table_info iteration in columnIsNotNull with a
single-row query against the pragma_table_info table-valued function, binding
both the table and column names. Note that the "notnull" result column must
be quoted, as NOTNULL is an SQLite operator keyword. Suggested by review, with
corrections for the keyword quoting and errorlint-compliant ErrNoRows check.

db/migrations/20260709160132_fix_bpm_bitdepth_not_null.go

@@ -0,0 +1,87 @@
+package migrations
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+
+	"github.com/navidrome/navidrome/log"
+	"github.com/pressly/goose/v3"
+)
+
+func init() {
+	goose.AddMigrationContext(upFixBpmBitdepthNotNull, downFixBpmBitdepthNotNull)
+}
+
+// upFixBpmBitdepthNotNull repairs databases where media_file.bpm/bit_depth are
+// still declared NOT NULL even though migration 20260612222838 (which made them
+// nullable) is recorded as applied. On those instances the scanner writes a NULL
+// bpm for tracks without a BPM tag and the write fails with
+// "NOT NULL constraint failed: media_file.bpm" (issue #5747). Because goose
+// considers the earlier migration already applied, it will never re-run it, so a
+// fresh migration is required. It re-inspects the column definition and only
+// rebuilds a column when it is still NOT NULL, making it a no-op on healthy DBs.
+func upFixBpmBitdepthNotNull(ctx context.Context, tx *sql.Tx) error {
+	repaired := false
+
+	// bpm carries the media_file_bpm index, which must be dropped before the
+	// column can be dropped and recreated after.
+	notNull, err := columnIsNotNull(ctx, tx, "media_file", "bpm")
+	if err != nil {
+		return err
+	}
+	if notNull {
+		if _, err = tx.ExecContext(ctx, `
+drop index if exists media_file_bpm;
+alter table media_file add column bpm_new integer;
+update media_file set bpm_new = nullif(bpm, 0);
+alter table media_file drop column bpm;
+alter table media_file rename column bpm_new to bpm;
+create index if not exists media_file_bpm on media_file (bpm);
+`); err != nil {
+			return err
+		}
+		repaired = true
+	}
+
+	notNull, err = columnIsNotNull(ctx, tx, "media_file", "bit_depth")
+	if err != nil {
+		return err
+	}
+	if notNull {
+		if _, err = tx.ExecContext(ctx, `
+alter table media_file add column bit_depth_new integer;
+update media_file set bit_depth_new = nullif(bit_depth, 0);
+alter table media_file drop column bit_depth;
+alter table media_file rename column bit_depth_new to bit_depth;
+`); err != nil {
+			return err
+		}
+		repaired = true
+	}
+
+	if repaired {
+		log.Warn(ctx, "Repaired media_file.bpm/bit_depth NOT NULL columns left over from an out-of-order migration (issue #5747)")
+	}
+	return nil
+}
+
+func downFixBpmBitdepthNotNull(ctx context.Context, tx *sql.Tx) error {
+	// No-op: re-adding the NOT NULL constraint would reintroduce the bug.
+	return nil
+}
+
+// columnIsNotNull reports whether the given column of the given table is declared
+// with a NOT NULL constraint. "notnull" must be quoted: NOTNULL is an SQLite
+// operator keyword.
+func columnIsNotNull(ctx context.Context, tx *sql.Tx, table, column string) (bool, error) {
+	var notNull int
+	err := tx.QueryRowContext(ctx, `SELECT "notnull" FROM pragma_table_info(?) WHERE name = ?`, table, column).Scan(&notNull)
+	if errors.Is(err, sql.ErrNoRows) {
+		return false, nil
+	}
+	if err != nil {
+		return false, err
+	}
+	return notNull == 1, nil
+}