-- cohorts + cohort_enrollments — replaces the old students.cohort TEXT column with a real
-- many-to-many-capable model (a student can move between cohorts over time; the old model
-- couldn't represent that at all).

CREATE TABLE IF NOT EXISTS cohorts (
  cohort_id     TEXT PRIMARY KEY,          -- slug, e.g. 'root-fall-2026'
  name          TEXT NOT NULL,
  code          TEXT UNIQUE,               -- optional short join-style code, distinct from cohort_id
  tier          TEXT,                      -- e.g. 'root' — free text, not an enum yet (YAGNI)
  start_date    DATE,
  end_date      DATE,
  status        TEXT NOT NULL DEFAULT 'active',  -- 'active' | 'archived'
  seat_target   INTEGER,
  created_at    TIMESTAMPTZ NOT NULL DEFAULT now(),
  updated_at    TIMESTAMPTZ NOT NULL DEFAULT now()
);

CREATE INDEX IF NOT EXISTS idx_cohorts_status ON cohorts(status);

CREATE TABLE IF NOT EXISTS cohort_enrollments (
  cohort_id         TEXT NOT NULL REFERENCES cohorts(cohort_id),
  student_id        TEXT NOT NULL REFERENCES students(student_id),
  enrollment_status TEXT NOT NULL DEFAULT 'enrolled',  -- 'enrolled' | 'removed'
  enrolled_at       TIMESTAMPTZ NOT NULL DEFAULT now(),
  PRIMARY KEY (cohort_id, student_id)
);

CREATE INDEX IF NOT EXISTS idx_enrollments_student ON cohort_enrollments(student_id);