fix(library): 마이그레이션 asyncpg 다중 statement 분리

asyncpg는 prepared statement에 다중 SQL 불가.
120(테이블) → 121(unique idx) → 122(parent idx) → 123(시드) 분리.

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

migrations/120_library_categories.sql

@@ -1,5 +1,5 @@
 -- 자료실 분류 독립 관리 테이블
-CREATE TABLE library_categories (
+CREATE TABLE IF NOT EXISTS library_categories (
     id          BIGSERIAL PRIMARY KEY,
     path        TEXT NOT NULL,
     name        TEXT NOT NULL,
@@ -8,11 +8,4 @@ CREATE TABLE library_categories (
     is_system   BOOLEAN NOT NULL DEFAULT FALSE,
     created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
     updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
-);
-
-CREATE UNIQUE INDEX uq_library_categories_path ON library_categories (path);
-CREATE INDEX idx_library_categories_parent ON library_categories (parent_path);
-
--- 시스템 분류: 미분류
-INSERT INTO library_categories (path, name, parent_path, depth, is_system)
-VALUES ('미분류', '미분류', NULL, 1, TRUE);
+)

migrations/121_library_categories_indexes.sql

@@ -0,0 +1 @@
+CREATE UNIQUE INDEX IF NOT EXISTS uq_library_categories_path ON library_categories (path)

migrations/122_library_categories_parent_idx.sql

@@ -0,0 +1 @@
+CREATE INDEX IF NOT EXISTS idx_library_categories_parent ON library_categories (parent_path)

migrations/123_library_categories_seed.sql

@@ -0,0 +1,3 @@
+INSERT INTO library_categories (path, name, parent_path, depth, is_system)
+VALUES ('미분류', '미분류', NULL, 1, TRUE)
+ON CONFLICT DO NOTHING