feat: add database topic group with 105 questions

- mysql-acid (35): undo/redo log, WAL, 崩溃恢复三阶段, 两阶段提交
- mysql-transaction-isolation (35): 四隔离级别, 三异常, Read View, 快照读/当前读, next-key lock
- redis-persistence (35): RDB/AOF/混合持久化, AOF 重写, vs binlog/redo log
- 每子主题: single_choice×10 + fill_blank×10 + short_answer×10 + code_reading×5
- 全部通过 question.schema.json 校验

topics/index.json

@@ -1,6 +1,6 @@
 {
   "version": "1.0.0",
-  "updated": "2026-09-04",
+  "updated": "2026-09-07",
   "topics": [
     {
       "slug": "qunar-ai-fullstack",
@@ -287,6 +287,58 @@
           }
         }
       ]
+    },
+    {
+      "slug": "database",
+      "name": "数据库",
+      "description": "MySQL 与 Redis 核心原理：ACID 与三大日志、事务隔离级别、MVCC、Redis 持久化机制",
+      "subtopics": [
+        {
+          "slug": "mysql-acid",
+          "name": "MySQL ACID 实现机制",
+          "description": "InnoDB 如何用日志与锁实现事务的原子性/一致性/隔离性/持久性：undo log、redo log、WAL、MVCC、锁",
+          "path": "topics/database/mysql-acid",
+          "stats": {
+            "total": 35,
+            "by_type": {
+              "single_choice": 10,
+              "fill_blank": 10,
+              "code_reading": 5,
+              "short_answer": 10
+            }
+          }
+        },
+        {
+          "slug": "mysql-transaction-isolation",
+          "name": "MySQL 事务与隔离机制",
+          "description": "四种隔离级别、脏读/不可重复读/幻读、MVCC Read View 选版本逻辑、Repeatable Read 与 Next-Key Lock",
+          "path": "topics/database/mysql-transaction-isolation",
+          "stats": {
+            "total": 35,
+            "by_type": {
+              "single_choice": 10,
+              "fill_blank": 10,
+              "code_reading": 5,
+              "short_answer": 10
+            }
+          }
+        },
+        {
+          "slug": "redis-persistence",
+          "name": "Redis 持久化与日志对比",
+          "description": "AOF/RDB 持久化机制、AOF 重写压缩原理、与 MySQL binlog/redo log/undo log 的对比（数据结构角度）",
+          "path": "topics/database/redis-persistence",
+          "stats": {
+            "total": 35,
+            "by_type": {
+              "single_choice": 10,
+              "fill_blank": 10,
+              "code_reading": 5,
+              "short_answer": 10
+            }
+          }
+        }
+      ]
     }
   ]
 }