feat(book): 공부도구 배선 — 노트/형광펜/암기카드(clause_study) + 책 리더 패널

migrations/380_clause_study.sql

@@ -0,0 +1,9 @@
+-- 380_clause_study.sql — 절-문서 공부도구(노트/형광펜/암기카드) 저장. FK 없음(documents 락 회피).
+CREATE TABLE IF NOT EXISTS clause_study (
+  id         bigserial PRIMARY KEY,
+  doc_id     bigint NOT NULL,
+  kind       text   NOT NULL,            -- 'note' | 'highlight' | 'card'
+  payload    jsonb  NOT NULL DEFAULT '{}',
+  created_at timestamptz NOT NULL DEFAULT now()
+);
+CREATE INDEX IF NOT EXISTS idx_clause_study_doc ON clause_study(doc_id, kind);