Adds seed_cohort_9p_ph1.py — creates N student accounts (default 10) and enrols
them all into one class (Greenfield Year 9 Physics 9P/Ph1) so there is a real
cohort to mark. The canonical timetable seeds enrol one student per year-band,
leaving every class with <=1 student.
Uses the same paths as the canonical seeds (auth admin create user, profiles +
institute_memberships upsert, POST /database/timetable/classes/{id}/students as
school admin). Idempotent. Self-contained HTTP (runs inside the ccapi container).
Verified on dev .94: 10 created + enrolled, 0 errors; 9P/Ph1 roster = 11;
physics teacher sees all 11 under as-user RLS.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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