feat(digest): Phase 4 Global News Digest (cluster-level batch summarization)

7일 rolling window 뉴스를 country × topic 2-level로 묶어 매일 04:00 KST 배치 생성.
search 파이프라인 미사용. documents → clustering → cluster-level LLM summarization → digest.

핵심 결정:
- adaptive threshold (0.75/0.78/0.80) + EMA centroid (α=0.7) + time-decay (λ=ln(2)/3)
- min_articles=3, max_topics=10/country, top-5 MMR diversity, ai_summary[:300] truncate
- cluster-level LLM only, drop금지 fallback (topic_label="주요 뉴스 묶음" + top member ai_summary[:200])
- importance_score country별 0~1 normalize + raw_weight_sum 별도 보존, max(score, 0.01) floor
- per-call timeout 25s + pipeline hard cap 600s
- DELETE+INSERT idempotent (UNIQUE digest_date), AIClient._call_chat 직접 호출 (client.py 수정 없음)

신규:
- migrations/101_global_digests.sql (2테이블 정규화)
- app/models/digest.py (GlobalDigest + DigestTopic ORM)
- app/services/digest/{loader,clustering,selection,summarizer,pipeline}.py
- app/workers/digest_worker.py (PIPELINE_HARD_CAP + CLI 진입점)
- app/api/digest.py (/latest, ?date|country, /regenerate, inline Pydantic)
- app/prompts/digest_topic.txt (JSON-only + 절대 금지 블록)

main.py 4줄: import 2 + scheduler add_job 1 + include_router 1.
plan: ~/.claude/plans/quiet-herding-tome.md

migrations/101_global_digests.sql

@@ -0,0 +1,57 @@
+-- Phase 4 Global News Digest
+-- 7일 rolling window 뉴스를 country × topic 2-level로 묶어 매일 새벽 4시 KST 배치 생성
+-- 검색 파이프라인 미사용. documents → clustering → cluster-level LLM summarization → digest
+-- 사용자 결정: country→topic 2-level, cluster-level LLM only, drop 금지 fallback,
+--             adaptive threshold, EMA centroid, time-decay (λ=ln(2)/3 ≈ 0.231)
+
+-- 부모 테이블: 하루 단위 digest run 메타데이터
+CREATE TABLE global_digests (
+    id              BIGSERIAL PRIMARY KEY,
+    digest_date     DATE NOT NULL,                    -- KST 기준 생성일
+    window_start    TIMESTAMPTZ NOT NULL,             -- rolling window 시작 (UTC)
+    window_end      TIMESTAMPTZ NOT NULL,             -- 생성 시점 (UTC)
+    decay_lambda    DOUBLE PRECISION NOT NULL,        -- 실제 사용된 time-decay λ
+
+    total_articles  INTEGER NOT NULL DEFAULT 0,
+    total_countries INTEGER NOT NULL DEFAULT 0,
+    total_topics    INTEGER NOT NULL DEFAULT 0,
+
+    generation_ms   INTEGER,                          -- 워커 실행 시간 (성능 회귀 감지)
+    llm_calls       INTEGER NOT NULL DEFAULT 0,
+    llm_failures    INTEGER NOT NULL DEFAULT 0,       -- = fallback 사용 횟수
+    status          VARCHAR(20) NOT NULL DEFAULT 'success',  -- success | partial | failed
+
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+
+    UNIQUE (digest_date)   -- idempotency: 같은 날짜 재실행 시 DELETE+INSERT
+);
+
+CREATE INDEX idx_global_digests_date ON global_digests (digest_date DESC);
+
+-- 자식 테이블: country × topic 단위
+CREATE TABLE digest_topics (
+    id                BIGSERIAL PRIMARY KEY,
+    digest_id         BIGINT NOT NULL REFERENCES global_digests(id) ON DELETE CASCADE,
+
+    country           VARCHAR(10) NOT NULL,           -- KR | US | JP | CN | FR | DE | ...
+    topic_rank        INTEGER NOT NULL,               -- country 내 1..N (importance_score 내림차순)
+
+    topic_label       TEXT NOT NULL,                  -- LLM 생성 5~10 단어 한국어 (또는 fallback 시 "주요 뉴스 묶음")
+    summary           TEXT NOT NULL,                  -- LLM 생성 1~2 문장 factual (또는 fallback 시 top member ai_summary[:200])
+
+    article_ids       JSONB NOT NULL,                 -- [doc_id, ...] 코드가 주입 (LLM 생성 금지)
+    article_count     INTEGER NOT NULL,               -- = jsonb_array_length(article_ids)
+
+    importance_score  DOUBLE PRECISION NOT NULL,      -- batch 내 country별 0~1 normalized (cross-country 비교)
+    raw_weight_sum    DOUBLE PRECISION NOT NULL,      -- 정규화 전 decay 가중합 (디버그 + day-over-day 트렌드)
+
+    centroid_sample   JSONB,                          -- 디버그: LLM 입력 doc id 목록 + summary hash
+    llm_model         VARCHAR(100),                   -- 사용된 모델 (primary/fallback 추적)
+    llm_fallback_used BOOLEAN NOT NULL DEFAULT FALSE, -- LLM 실패 시 minimal fallback 적용 여부
+
+    created_at        TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_digest_topics_digest ON digest_topics (digest_id);
+CREATE INDEX idx_digest_topics_country ON digest_topics (country);
+CREATE INDEX idx_digest_topics_rank ON digest_topics (digest_id, country, topic_rank);