feat: study cohorts — group enrollment with shared progress & leaderboard

[zeemscript]

Summary

Implements #1027 — study cohorts (squads): small groups of learners enrolled in the same track who share a group progress view, a private discussion thread, and a group leaderboard. Closes #1027.

What's included

Database

• Migration 025_study_cohorts.sql (+ .undo.sql): cohorts and cohort_members tables per the issue spec, with indexes on course_slug / learner_addr and a max_members > 0 check.

Backend (createCohortsRouter, mounted under /api)

• POST /api/cohorts — create (auth + zod validation); validates the course exists; creator auto-joins in a transaction.
• GET /api/cohorts?course= — list cohorts with live member counts.
• POST /api/cohorts/:id/join — capacity-checked inside a transaction with a SELECT ... FOR UPDATE row lock so concurrent joins can't oversubscribe; idempotent when already a member.
• POST /api/cohorts/:id/leave — idempotent no-op when not a member.
• GET /api/cohorts/:id — members with per-member progress joined against milestone_reports (approved only) plus a group completion %; ordered by milestones completed so the payload doubles as the leaderboard. Falls back to lesson count for courses without milestone rows.

Frontend

• New Squads tab on the course page (LessonView, alongside Lesson/Forum).
• SquadsPanel: browse/join cohorts for the course or create one.
• CohortDetailView: member roster with progress bars, group completion ring, ranked leaderboard (crown for Bump the npm-minor group with 11 updates #1, "You" marker), and an embedded discussion thread reusing CommentSection scoped to thread id cohort-<id> (members only).
• React Query hooks in src/hooks/useCohorts.ts using the shared apiFetchJson wrapper.

Tests

• server/src/tests/cohorts-api.test.ts (16 tests): capacity enforcement, join/leave idempotency, group progress aggregation, auth/validation, course-not-found.
• src/components/cohorts/CohortDetailView.test.tsx (4 tests): ring/roster rendering, leaderboard "You" marker, cohort-scoped discussion, full-squad join disabled.

Repo fix (needed to build)

• package-lock.json had a corrupted entry (node_modules/esbuild/node_modules/@esbuild/aix-ppc64 with no version field) that made npm install / npm ci fail with npm error Invalid Version:. Removed the stub so npm resolves the proper top-level entry.

Acceptance criteria

• A learner can create and join a cohort for a course
• Cohort detail shows real per-member progress (approved milestone_reports)
• Capacity is enforced; can't join a full cohort (409, race-safe)
• Discussion thread scoped to the cohort works (reuses CommentSection)

Verification

• Server: 16 new Jest/supertest tests pass; tsc --noEmit clean.
• Frontend: 4 new vitest tests pass; vite build succeeds; no new eslint errors.
• Pre-existing LessonView.test.tsx failures are identical on pristine main (18 failed) — unrelated to this change.

[Anuoluwapo25]

please fix checks

[Anuoluwapo25]

Hi @zeemscript, Just want to confirm if you are still working on this issue