Fix missing prerequisite edges for units using enrolment_rules

packages/db/drizzle/0007_backfill_enrolment_rule_refs.sql

@@ -0,0 +1,38 @@
+-- Backfill requisite_refs for units that store their PREREQUISITE /
+-- PROHIBITION relationships in enrolment_rules HTML rather than in the
+-- structured requisites field.
+--
+-- Pattern: entries whose description contains a <strong>PREREQUISITE</strong>
+-- or <strong>PROHIBITION</strong> label followed by handbook unit links.
+-- We extract every /units/CODE href from those descriptions and insert a
+-- matching requisite_refs edge.  ON CONFLICT is a no-op so this is safe
+-- to re-run, and structured-requisite rows are never duplicated because
+-- the two sources are disjoint (verified: zero overlap in 2026 corpus).
+
+--> statement-breakpoint
+
+-- Prerequisites from enrolment_rules HTML
+INSERT INTO requisite_refs (year, unit_code, requisite_type, requires_unit_code)
+SELECT DISTINCT
+  er.year,
+  er.unit_code,
+  'prerequisite'::requisite_type,
+  upper(m[1])
+FROM enrolment_rules er,
+     regexp_matches(er.description, 'handbook\.monash\.edu/[^"]+/units/([A-Za-z][A-Za-z0-9-]*)', 'g') m
+WHERE er.description ~* '<strong>\s*PREREQUISITE'
+ON CONFLICT (year, unit_code, requisite_type, requires_unit_code) DO NOTHING;
+
+--> statement-breakpoint
+
+-- Prohibitions from enrolment_rules HTML
+INSERT INTO requisite_refs (year, unit_code, requisite_type, requires_unit_code)
+SELECT DISTINCT
+  er.year,
+  er.unit_code,
+  'prohibition'::requisite_type,
+  upper(m[1])
+FROM enrolment_rules er,
+     regexp_matches(er.description, 'handbook\.monash\.edu/[^"]+/units/([A-Za-z][A-Za-z0-9-]*)', 'g') m
+WHERE er.description ~* '<strong>\s*PROHIBITION'
+ON CONFLICT (year, unit_code, requisite_type, requires_unit_code) DO NOTHING;