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/database/mysql-acid/meta.json

@@ -0,0 +1,50 @@
+{
+  "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",
+  "tags": [
+    "ACID",
+    "InnoDB",
+    "MVCC",
+    "WAL",
+    "binlog",
+    "innodb_flush_log_at_trx_commit",
+    "next-key lock",
+    "redo log",
+    "undo log",
+    "一致性",
+    "两阶段提交",
+    "原子性",
+    "崩溃恢复",
+    "快照读",
+    "性能",
+    "持久性",
+    "物理日志",
+    "约束",
+    "行锁",
+    "逻辑日志",
+    "间隙锁",
+    "隔离性"
+  ],
+  "difficulty_range": [
+    2,
+    4
+  ],
+  "schema_version": "1.0.0",
+  "updated": "2026-09-07",
+  "question_files": [
+    "single_choice",
+    "fill_blank",
+    "code_reading",
+    "short_answer"
+  ],
+  "stats": {
+    "total": 35,
+    "by_type": {
+      "single_choice": 10,
+      "fill_blank": 10,
+      "code_reading": 5,
+      "short_answer": 10
+    }
+  }
+}