v1.3.4: 통계 확장(체류·스크롤·실시간 접속자)

- 031 마이그레이션: 체류·스크롤 집계, analytics_active_sessions
- heartbeat API, 관리자 realtime API, 클라이언트 heartbeat
- 대시보드: 현재 접속자 목록(로그인 닉네임·아바타), 참여 지표

db/migrations/031_analytics_engagement_and_realtime.sql

@@ -0,0 +1,30 @@
+ALTER TABLE site_analytics_daily
+  ADD COLUMN IF NOT EXISTS engaged_views INTEGER NOT NULL DEFAULT 0,
+  ADD COLUMN IF NOT EXISTS total_engaged_seconds INTEGER NOT NULL DEFAULT 0;
+
+ALTER TABLE post_analytics_daily
+  ADD COLUMN IF NOT EXISTS engaged_views INTEGER NOT NULL DEFAULT 0,
+  ADD COLUMN IF NOT EXISTS total_engaged_seconds INTEGER NOT NULL DEFAULT 0,
+  ADD COLUMN IF NOT EXISTS scroll_25 INTEGER NOT NULL DEFAULT 0,
+  ADD COLUMN IF NOT EXISTS scroll_50 INTEGER NOT NULL DEFAULT 0,
+  ADD COLUMN IF NOT EXISTS scroll_75 INTEGER NOT NULL DEFAULT 0,
+  ADD COLUMN IF NOT EXISTS scroll_100 INTEGER NOT NULL DEFAULT 0;
+
+CREATE TABLE IF NOT EXISTS analytics_active_sessions (
+  session_hash TEXT PRIMARY KEY,
+  user_id UUID REFERENCES users(id) ON DELETE SET NULL,
+  path TEXT NOT NULL,
+  post_id UUID REFERENCES posts(id) ON DELETE SET NULL,
+  post_slug TEXT NOT NULL DEFAULT '',
+  first_seen_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+  last_seen_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+  duration_seconds INTEGER NOT NULL DEFAULT 0,
+  max_scroll_ratio REAL NOT NULL DEFAULT 0
+);
+
+CREATE INDEX IF NOT EXISTS analytics_active_sessions_last_seen_idx
+  ON analytics_active_sessions (last_seen_at DESC);
+
+CREATE INDEX IF NOT EXISTS analytics_active_sessions_user_idx
+  ON analytics_active_sessions (user_id)
+  WHERE user_id IS NOT NULL;