Contact Rollups: remove legacy pre-2017 PD attendance extraction - #74614

What

Deletes the nightly Contact Rollups extraction over the pre-2017 PD attendance model, plus its processing code.

Before Sept 2017, PD workshop attendance was modeled as sections rows with a non-null section_type joined through followers. Commit 7d3b6ab ("Remove all workshop.section references, switch completely to the new attendance model", 2017-09-06) moved attendance entirely to pd_attendances, and nothing has written a workshop section since — the matching rows have been frozen for eight years. The nightly job still re-scanned users ⨝ followers ⨝ sections (no index on section_type; followers is one of the largest tables in the database) every single run to re-extract them.

Marketing confirmed that contacts whose only PD attendance predates the 2017 model are no longer relevant, so this is a delete, not a snapshot.

• Remove extract_professional_learning_attendance_old_attendance_model and its call in collect_contacts.
• Rename extract_professional_learning_attendance_new_attendance_model → extract_professional_learning_attendance, since there is only one model now.
• Remove the dashboard.followers handling in ContactRollupsProcessed: the section_type → course mapping (SECTION_TYPE_INVERTED_MAP) in extract_professional_learning_attended, and the dashboard.followers key check in extract_roles' Teacher rule.

Expected Pardot effect (one-time)

• Teachers whose attendance list included old-model courses get one update shortening db_Professional_Learning_Attended on the next nightly run.
• Pardot multi-value fields sync as indexed keys (db_X_0, db_X_1, …) and the delta calculation only sends keys present in new data, so when a list shrinks, trailing indexed values are not cleared in Pardot. Those stale tail values are a known, accepted one-time artifact of dropping this data.
• Teachers whose only teacher signal was an old-model followers row lose that source for the Teacher role; any surviving role change syncs the same way.

Performance

Removes one of the heaviest collect-phase queries (full scan of sections filtered on unindexed section_type, joined to followers and users) and the corresponding raw-row volume from every downstream step (aggregation, parsing, sync joins).

Testing

• Updated contact_rollups_processed_test.rb: extract_professional_learning_attended cases now exercise pd_attendances only (including dedup/compact/sort); extract_roles no longer expects a followers-based Teacher role.
• 32 tests pass across the three contact-rollups test files; the single error in contact_rollups_raw_test.rb (teacher-verification/...png missing from the local test asset pipeline) is a pre-existing local-environment issue, unchanged by this diff.
• Lint (pre-commit hook) clean.

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