perf: optimize EventType children count query using raw SQL

[keithwillcode]

What does this PR do?

Optimizes a slow SQL query that counts EventType children. The original Prisma-generated query was doing a full table sequential scan (~3.5M rows, 825ms execution time) because the _count.children aggregation generates a subquery with WHERE 1=1 that scans ALL rows.

The new approach uses a raw SQL query that filters the subquery to only count children for the specific IDs being queried, leveraging the existing parentId index.

Before (Prisma-generated):

SELECT ... COALESCE("aggr_selection_0_EventType"."_aggr_count_children", 0) 
FROM "public"."EventType" 
LEFT JOIN (
  SELECT "parentId", COUNT(*) FROM "public"."EventType" 
  WHERE 1=1  -- Full table scan!
  GROUP BY "parentId"
) ...

After (optimized raw SQL):

SELECT ... COALESCE(child_counts.children_count, 0) 
FROM "public"."EventType" et
LEFT JOIN (
  SELECT "parentId", COUNT(*) FROM "public"."EventType" 
  WHERE "parentId" = ANY($1::int[])  -- Uses parentId index
  GROUP BY "parentId"
) ...

This count is used only for displaying "(+N)" in workflow list tooltips to show child event type counts.

Updates since last revision

• Added missing await to all enrichWorkflowsWithChildrenCount calls in WorkflowRepository
• Reverted eventTypeRepository changes - the optimization is now only applied in WorkflowRepository. The eventTypeRepository methods (findById, findByIdForOrgAdmin) continue to use Prisma's _count.children because the tRPC types expect this structure and these are single event type queries with minimal performance impact.

Link to Devin run: https://app.devin.ai/sessions/be18fff405b642a7aa33be8bf5ca5146
Requested by: @keithwillcode

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A - no documentation changes needed.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

1. Navigate to the Workflows page where workflows are listed with "Active on X event types"
2. Hover over the event types to see the tooltip with child counts (e.g., "30 Min Meeting (+3)")
3. Verify the counts display correctly
4. For performance verification: Check database query logs to confirm the new query uses an index scan instead of sequential scan

Checklist for Human Review

• Verify the raw SQL query produces identical results to the original Prisma query
• Check if hardcoded "public"."EventType" schema is appropriate for all environments
• Review the as EnrichActiveOn<T>[] type cast in enrichWorkflowsWithChildrenCount
• Verify BigInt to Number conversion handles edge cases correctly

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