Function execute_aggregate 

pub async fn execute_aggregate(
    registry: &TableRegistry,
    sources: SourceSpec,
    filter: Option<ListFilter>,
    aggregator: AliasAggregator,
    schema: &SchemaConfig,
) -> Result<AliasRunResult, MiniAppError>

Execute a multi-table aggregation request.

Resolves each source table via registry, mounts every backing .db file into a fresh in-memory connection with ATTACH DATABASE, composes a UNION ALL inner sub-query (or a single SELECT for SourceSpec::Single), and wraps it in an outer SELECT <aggregate> (with GROUP BY + optional HAVING for AliasAggregator::GroupBy).

Crux compliance

• Crux #1: reuses the existing ListFilter::build_sql via the sibling ListFilter::build_subquery method (no signature change).
• Crux #2: having is emitted as a literal HAVING clause after GROUP BY, never as WHERE.
• Crux #3: multi-table sources are combined with literal UNION ALL; JOIN is never emitted and no application-layer merge happens.

Errors

• MiniAppError::Aggregator — empty Multi, ATTACH-limit exceeded, nested GroupBy, or non-UTF-8 db path.
• MiniAppError::Validation — schema field missing, identifier regex mismatch, or filter validation failure.
• MiniAppError::TableNotFound — propagated from the registry.
• MiniAppError::Storage — rusqlite failure (ATTACH, query execution, etc.).
• MiniAppError::Schema — spawn_blocking panic.