Add the ticket_metrics stream to the Zendesk Airbyte connector #2539

Description/Context

Exactly seven Zendesk streams are landed today (src/ol_dbt/models/staging/zendesk/_zendesk__sources.yml): tickets, ticket_comments, ticket_fields, brands, groups, organizations, users.

solved_at, closed_at, initially_assigned_at, first_resolution_time and full_resolution_time live in Zendesk's ticket_metrics endpoint; full state history lives in ticket_audits. Neither is synced. In the tickets stream, "solved"/"closed" appear only as values of ticket_status — a snapshot with no history, so resolution time is currently underivable.

Plan/Design

Adding ticket_metrics unblocks the currently-blocked columns on afact_feedback_conversation: first_response_date_fk, resolved_date_fk, closed_date_fk, resolution_duration_seconds.

Non-blocking for the MVP — the conversation fact ships without them and its grain does not move when they arrive, so this lands additively whenever the ingestion change is convenient. Worth doing because "how long was this open" is a natural first question for the support audience.

Spec: feedback_dimensional_model.md §5a