feat(library): Phase 2A facet 탐색 기반 — 컬럼 + API + 필터

documents 테이블에 facet_company/topic/year/doctype 4개 축 추가.
facet_values 사전 테이블 + CRUD API.
facet-counts 집계 API (교차 필터링 지원).
문서 목록 API에 facet 필터 파라미터 추가.
DocumentResponse/DocumentUpdate 스키마에 facet 필드 포함.

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

migrations/124_facet_columns.sql

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+ALTER TABLE documents ADD COLUMN IF NOT EXISTS facet_company TEXT

migrations/125_facet_topic.sql

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+ALTER TABLE documents ADD COLUMN IF NOT EXISTS facet_topic TEXT

migrations/126_facet_year.sql

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+ALTER TABLE documents ADD COLUMN IF NOT EXISTS facet_year INT

migrations/127_facet_doctype.sql

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+ALTER TABLE documents ADD COLUMN IF NOT EXISTS facet_doctype TEXT

migrations/128_facet_indexes.sql

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+CREATE INDEX IF NOT EXISTS idx_documents_facet_company ON documents (facet_company) WHERE facet_company IS NOT NULL

migrations/129_facet_topic_idx.sql

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+CREATE INDEX IF NOT EXISTS idx_documents_facet_topic ON documents (facet_topic) WHERE facet_topic IS NOT NULL

migrations/130_facet_year_idx.sql

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+CREATE INDEX IF NOT EXISTS idx_documents_facet_year ON documents (facet_year) WHERE facet_year IS NOT NULL

migrations/131_facet_doctype_idx.sql

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+CREATE INDEX IF NOT EXISTS idx_documents_facet_doctype ON documents (facet_doctype) WHERE facet_doctype IS NOT NULL

migrations/132_facet_values.sql

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+CREATE TABLE IF NOT EXISTS facet_values (
+    id          BIGSERIAL PRIMARY KEY,
+    facet_type  TEXT NOT NULL,
+    value       TEXT NOT NULL,
+    is_system   BOOLEAN NOT NULL DEFAULT FALSE,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+)

migrations/133_facet_values_unique.sql

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+CREATE UNIQUE INDEX IF NOT EXISTS uq_facet_values_type_value ON facet_values (facet_type, value)