feat(graphql): groupBy + aggregate functions on Connection types

[rubenvdlinde]

Context

The @conduction/nextcloud-vue library is adding manifest-driven dashboard widgets that pull aggregated data from OR's GraphQL endpoint (see ConductionNL/nextcloud-vue PR-A — manifest dataSource block on widget definitions). The first cut covers `stats-block` widgets via the existing `Connection.totalCount` field, which works today.

Charts and breakdowns need richer aggregation that OR's GraphQL doesn't expose yet.

Deferred from

Library-side widget dispatcher work (PR-A) intentionally stops at `totalCount`. Chart widgets currently render static `series` / `categories` from the manifest; the lib's new `dataSource.graphql.selectors` form supports any custom query, but there's no server endpoint to query against until this lands.

Ask

Add to OR's generated GraphQL schema:

1. `groupBy: [String!]` argument on connection list fields. Buckets results by the named property paths (dot-paths into JSON columns). Returns a list of group buckets instead of `edges`.
2. `aggregate` selection set on each bucket — `{ count, sum(field), avg(field), min(field), max(field) }`.
3. Optional `having` filter on aggregated values.

Example target query (decidesk meeting status breakdown):
```graphql
{
meetingsList(filter: {}, groupBy: ["lifecycle"]) {
groups {
key { lifecycle }
aggregate { count }
}
}
}
```

Pickup trigger

• Decidesk / mydash chart widgets need server-driven series.
• Procest / pipelinq KPI breakdowns (currently client-aggregated) get migrated.

Notes

• Connection `totalCount` already exists (SchemaGenerator/TypeMapperHandler.php:493).
• Existing list args at TypeMapperHandler.php:575 are the place to extend with `groupBy`.
• See `graphql-php`'s standard pattern for connection aggregations (Hasura / DGS / Postgraphile) for the field shape.