perf(dbt): optimize tableau gradebook and topline weekly materializations

[cbini]

Problem

BigQuery costs increased ~62% ($278 → $451) from February to March 2026. Two model groups account for ~$80/month of the $173 increase:

1. Tableau gradebook models (+$53/month): 4 rpt_tableau__gradebook_* tables rebuilt ~100x/day, consumed 1x/day
2. Topline weekly models (+$27/month): 6 int_topline__*_weekly tables rebuilt ~130x/day, consumed 1x/day

Design

See spec: docs/superpowers/specs/2026-04-06-dbt-materialization-cost-optimization-design.md

Three phases:

1. Split int_tableau__gradebook_audit_flags by granularity (student/teacher), materialize the split tables, revert rpt views
2. Materialize int_extracts__student_enrollments_weeks spine, revert topline weekly models to views, fix duplicate windows
3. (Future) Reduce Dagster materialization frequency