fix(database): strip SQL comments before parsing and add expires_at column

The schema migration parser failed to recognize CREATE INDEX statements
preceded by SQL comments, causing duplicate index errors on repeated
bootstrap. Strip leading comment lines before pattern matching.

Also adds the missing expires_at column to user_api_keys via migration
000004, fixing Unknown column errors in API key creation.

Author: GALIAIS

backend/internal/database/migrations/000004_api_key_expires_at.sql

@@ -0,0 +1 @@
+ALTER TABLE user_api_keys ADD COLUMN expires_at DATETIME NULL DEFAULT NULL;

backend/internal/database/schema.go

@@ -63,19 +63,37 @@ func splitSQLStatements(body string) []string {
 }
 
 func applySchemaStatement(ctx context.Context, db *gorm.DB, statement string) error {
-	if indexName, tableName, ok := parseCreateIndex(statement); ok {
-		return ensureIndex(ctx, db, tableName, indexName, statement)
+	cleaned := stripSQLComments(statement)
+	if cleaned == "" {
+		return nil
+	}
+
+	if indexName, tableName, ok := parseCreateIndex(cleaned); ok {
+		return ensureIndex(ctx, db, tableName, indexName, cleaned)
 	}
-	if tableName, columnName, ok := parseAlterAddColumn(statement); ok {
-		return ensureColumn(ctx, db, tableName, columnName, statement)
+	if tableName, columnName, ok := parseAlterAddColumn(cleaned); ok {
+		return ensureColumn(ctx, db, tableName, columnName, cleaned)
 	}
 
-	if err := db.WithContext(ctx).Exec(statement).Error; err != nil {
+	if err := db.WithContext(ctx).Exec(cleaned).Error; err != nil {
 		return fmt.Errorf("apply schema statement: %w", err)
 	}
 	return nil
 }
 
+func stripSQLComments(statement string) string {
+	lines := strings.Split(statement, "\n")
+	filtered := make([]string, 0, len(lines))
+	for _, line := range lines {
+		trimmed := strings.TrimSpace(line)
+		if trimmed == "" || strings.HasPrefix(trimmed, "--") {
+			continue
+		}
+		filtered = append(filtered, line)
+	}
+	return strings.TrimSpace(strings.Join(filtered, "\n"))
+}
+
 func parseCreateIndex(statement string) (string, string, bool) {
 	matches := createIndexPattern.FindStringSubmatch(strings.TrimSpace(statement))
 	if len(matches) != 3 {