-- 381_study_concept_progress.sql — 이론 개념(문서) 간격반복(SR) 진행. 이론공부 홈 트리거.
-- concept_doc_id 는 documents.id 를 가리키나 FK 미설정(hot 테이블 락 회피, clause_study 380 선례).
-- SR 산술은 study_question_progress 와 동일(sr_schedule 공용): stage 0→1→2→3(1·3·7·14일)→4 졸업.
CREATE TABLE IF NOT EXISTS study_concept_progress (
  id             bigserial PRIMARY KEY,
  user_id        bigint NOT NULL REFERENCES users(id) ON DELETE CASCADE,
  study_topic_id bigint NOT NULL REFERENCES study_topics(id) ON DELETE CASCADE,
  concept_doc_id bigint NOT NULL,
  review_stage   smallint,
  due_at         timestamptz,
  last_read_at   timestamptz,
  created_at     timestamptz NOT NULL DEFAULT now(),
  updated_at     timestamptz NOT NULL DEFAULT now(),
  CONSTRAINT uq_concept_progress_user_doc UNIQUE (user_id, concept_doc_id)
);
CREATE INDEX IF NOT EXISTS idx_concept_progress_due ON study_concept_progress(user_id, due_at) WHERE due_at IS NOT NULL;