perf: 重构 FTS 索引为 rowid 寻址+触发器维护，根治重启 CPU 占满与扫描卡死

- FTS5 普通列(含 UNINDEXED)不支持索引查找,旧版按 media_id 做
  NOT EXISTS/DELETE 全是整表扫描:启动回填 O(N^2) 烧 CPU 数小时,
  扫描时每个 upsert 一次全表扫描,均隔着全局写锁拖死登录(125s 超时)
- v2 布局:FTS rowid 与 media.rowid 对齐,由 INSERT/UPDATE/DELETE
  触发器实时维护;回填只剩 rowid 点查兜底,且去掉 ORDER BY
- 顺带修复刮削直写 Updates() 后新标题搜不到的问题(触发器覆盖)
- 写门改为 context 感知:长写语句不再让登录请求无限期排队
- warmMediaSearchIndex 延迟 30s 错峰启动
- library_scan 周期任务跳过云盘库(由 cloud_sync 夜间窗口负责),
  首轮延迟从 15s 改为等满一个周期,重启不再立即全量扫描风暴
- prune 改为只取 id/path 并按 500 条批量删除,缩短写锁占用

internal/repository/repository_test.go

@@ -153,12 +153,17 @@ func TestMediaSearchIndexBackfillRunsInBatches(t *testing.T) {
 	}).Error; err != nil {
 		t.Fatal(err)
 	}
-	var before int64
-	if err := repos.DB.Raw(`SELECT COUNT(*) FROM media_search_fts`).Scan(&before).Error; err != nil {
+	// 插入触发器应当同步维护 FTS 行。
+	var indexed int64
+	if err := repos.DB.Raw(`SELECT COUNT(*) FROM media_search_fts`).Scan(&indexed).Error; err != nil {
 		t.Fatal(err)
 	}
-	if before != 0 {
-		t.Fatalf("startup migrate should not synchronously backfill FTS, got %d rows", before)
+	if indexed != 1 {
+		t.Fatalf("insert trigger should index new media, got %d rows", indexed)
+	}
+	// 清空 FTS 模拟旧库升级后索引缺失，回填应按批补齐且 rowid 对齐。
+	if err := repos.DB.Exec(`DELETE FROM media_search_fts`).Error; err != nil {
+		t.Fatal(err)
 	}
 	n, err := repos.Media.BackfillSearchIndex(t.Context(), 1)
 	if err != nil {
@@ -167,11 +172,22 @@ func TestMediaSearchIndexBackfillRunsInBatches(t *testing.T) {
 	if n != 1 {
 		t.Fatalf("backfilled rows = %d, want 1", n)
 	}
+	var aligned int64
+	if err := repos.DB.Raw(`SELECT COUNT(*) FROM media_search_fts f JOIN media m ON f.rowid = m.rowid AND f.media_id = m.id`).Scan(&aligned).Error; err != nil {
+		t.Fatal(err)
+	}
+	if aligned != 1 {
+		t.Fatalf("fts rows aligned with media rowid = %d, want 1", aligned)
+	}
+	// 软删除后触发器应清理对应 FTS 行，避免搜索命中已删媒体。
+	if err := repos.DB.Delete(&model.Media{}, "id = ?", "m-backfill").Error; err != nil {
+		t.Fatal(err)
+	}
 	var after int64
 	if err := repos.DB.Raw(`SELECT COUNT(*) FROM media_search_fts`).Scan(&after).Error; err != nil {
 		t.Fatal(err)
 	}
-	if after != 1 {
-		t.Fatalf("fts rows = %d, want 1", after)
+	if after != 0 {
+		t.Fatalf("soft delete should drop fts row, got %d", after)
 	}
 }