588951 Query Deployment Count Performance

What does this MR do and why?

Fixes intermittent timeouts on the deploymentCount GraphQL field for groups with many projects and environments (related to #588951).

The existing Postgres query against dora_daily_metrics takes ~1.3 minutes in production. This MR adds a ClickHouse-backed alternative that runs in ~0.4 seconds by querying siphon_deployments and siphon_environments directly using traversal path filtering.

Existing query (~1.3 minutes): https://postgres.ai/console/gitlab/gitlab-production-main/sessions/50482/commands/149922.

New CH query: 0.41 seconds

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Related to #588951