External Dashboards API: support container repeat field #2213

Context

PR #2201 wires the basic dashboard containers and tabs shape through the v2 external API. The container-level repeat field (which expands a single container into one instance per group-by value at render time) was called out in #2015 review notes as future work and is not yet exposed.

Scope

• Add repeat to DashboardContainerSchema in @hyperdx/common-utils if not already there (the UI already uses it).
• Mirror it on the v2 external API body schema and OpenAPI JSDoc.
• Conversion utilities in packages/api/src/routers/external-api/v2/utils/dashboards.ts propagate the field on create / get / update.
• Integration tests in packages/api/src/routers/external-api/__tests__/dashboards.test.ts cover round-trip with repeat set, and the empty case (omitted) round-trips as before.

Out of scope

• The render-time expansion logic in the app is unchanged. This issue is purely about the API surface.
• MCP is tracked separately as part of the containers/tabs MCP follow-up.

Acceptance

• API consumers can set repeat on a container and read it back unchanged.
• OpenAPI spec lists the field with constraint parity to the Zod schema.