Complete GORM to sqlc migration

- Remove GORM dependency, use sqlc for type-safe SQL queries
- Update all handlers to use sqlc patterns (context, value types)
- Fix N+1 query problem in domain listing with JOIN query
- Enable SQL query logging in debug mode (USE_EMBEDDED=false)
- Add comprehensive comments for non-Go developers

backend/internal/db/virtual_domains.go

@@ -43,3 +43,11 @@ func (d *DB) GetAllDomainsWithCounts(ctx context.Context) ([]DomainStats, error)
 	}
 	return stats, nil
 }
+
+func (d *DB) CountUsersByDomain(ctx context.Context, domainID uint32) (int64, error) {
+	return d.Queries.CountUsersByDomain(ctx, domainID)
+}
+
+func (d *DB) CountAliasesByDomain(ctx context.Context, domainID uint32) (int64, error) {
+	return d.Queries.CountAliasesByDomain(ctx, domainID)
+}