fix(queue): enqueue 경로 중복 방어 — partial unique index + 중앙 enqueue_stage 함수

기존 UNIQUE(document_id, stage, status)는 pending+processing 동시 존재를
허용해서 stale 복구 시 충돌 발생. 2-layer 방어로 근본 차단:

1) DB: partial unique index uq_queue_active — 활성 행(pending/processing)은
   (document_id, stage)당 최대 1개만 허용
2) App: enqueue_stage() 중앙 함수 — INSERT ON CONFLICT DO NOTHING으로
   모든 9개 경로의 check-then-insert TOCTOU race 제거

migration 117은 guard check 포함 — 활성 중복이 남아있으면 RAISE EXCEPTION
으로 중단, 수동 정리 유도.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

migrations/117_queue_active_unique.sql

@@ -0,0 +1,36 @@
+-- Step 1: stale duplicate 삭제 (processing 10분+ 방치 + 같은 doc/stage에 pending 존재)
+DELETE FROM processing_queue a
+USING processing_queue b
+WHERE a.document_id = b.document_id
+  AND a.stage = b.stage
+  AND a.status = 'processing'
+  AND a.started_at < NOW() - INTERVAL '10 minutes'
+  AND b.status = 'pending';
+
+-- Step 2: guard check — 활성 중복이 남아있으면 migration 중단
+DO $$
+DECLARE
+  dup_count INTEGER;
+BEGIN
+  SELECT COUNT(*) INTO dup_count
+  FROM (
+    SELECT document_id, stage
+    FROM processing_queue
+    WHERE status IN ('pending', 'processing')
+    GROUP BY document_id, stage
+    HAVING COUNT(*) > 1
+  ) sub;
+
+  IF dup_count > 0 THEN
+    RAISE EXCEPTION 'migration 117 blocked: % active duplicate(s) remain. Run manual cleanup first.', dup_count;
+  END IF;
+END $$;
+
+-- Step 3: 기존 constraint 제거
+ALTER TABLE processing_queue
+    DROP CONSTRAINT processing_queue_document_id_stage_status_key;
+
+-- Step 4: partial unique index (활성 행은 (document_id, stage)당 최대 1개)
+CREATE UNIQUE INDEX uq_queue_active
+    ON processing_queue (document_id, stage)
+    WHERE status IN ('pending', 'processing');