Relation counts and filters: _count, some, every, none #80

Signal

As part of the relational API, we want:

• Relation counts in a select: { tourCount: count(rel.tours) }.
• Relation filters: exists / notExists helpers with correlated subqueries that read like some, every and none.

Why (MyRemotely)

• 104 _count uses and 39 some / every / none filters in our production code.
• Drizzle needs raw sql or manual subqueries for most of these.

Part of #52: the CAFE-stack roadmap signal from MyRemotely. This shows the direction we want, and it is not a demand. We can send a PR if you accept the idea.