feat(search): add domain_bucket rollup column (migration 377)

ai_domain(반자유 AI 분류, 드리프트)을 검색 스코프용 7버킷으로 결정적 롤업하는
STORED generated column. News 86% 기본제외 + 도메인 스코프 검색의 토대.
축: ai_domain(routing) 롤업 — category(UI) 아님.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

migrations/377_domain_bucket.sql

@@ -0,0 +1,23 @@
+-- 377_domain_bucket.sql
+-- ai_domain(반자유 AI 분류, 드리프트 존재)을 검색 스코프용 7버킷으로 결정적 롤업.
+-- 축: ai_domain(routing/해석 축)의 coarsening — category(UI축) 아님 (feedback_category_vs_ai_domain_axis 준수).
+-- 버킷: News / Safety / Law / Engineering / General / Philosophy / Programming.
+-- STORED generated → 신규/재분류 문서도 ai_domain 붙으면 자동 버킷. ai_domain 원본 보존(하위 검색 유지).
+-- 롤백: ALTER TABLE documents DROP COLUMN domain_bucket;
+ALTER TABLE documents ADD COLUMN IF NOT EXISTS domain_bucket text
+  GENERATED ALWAYS AS (
+    CASE
+      WHEN ai_domain LIKE 'News%' THEN 'News'
+      WHEN ai_domain = '법령' OR ai_domain LIKE 'Industrial_Safety/Legislation%' THEN 'Law'
+      WHEN ai_domain = 'Safety' OR ai_domain LIKE 'Safety/%'
+           OR ai_domain LIKE 'Industrial_Safety%'
+           OR ai_domain = 'Knowledge/Industrial_Safety' THEN 'Safety'
+      WHEN ai_domain LIKE 'Engineering%' OR ai_domain = 'Knowledge/Engineering' THEN 'Engineering'
+      WHEN ai_domain LIKE 'Philosophy%' THEN 'Philosophy'
+      WHEN ai_domain LIKE 'Programming%' THEN 'Programming'
+      ELSE 'General'
+    END
+  ) STORED;
+
+CREATE INDEX IF NOT EXISTS documents_domain_bucket_idx
+  ON documents (domain_bucket) WHERE deleted_at IS NULL;