Please address the comments from this code review:

## Overall Comments
- The new platform stats aggregation logic (`get_platform_stats` and `get_platform_stats_time_series`) is now duplicated between SQLite and MySQL with only SQL dialect differences; consider extracting the common aggregation shape to a shared helper to reduce drift and keep behavior aligned across databases.
- In `mysql.py`, `NOT_GIVEN` is defined as a `TypeVar` but used as a runtime sentinel in `update_persona`; it would be clearer and more conventional to use a unique object instance (e.g. `NOT_GIVEN = object()`) for this purpose.
- The migration helper now short-circuits for `DatabaseType.MYSQL`, meaning any existing SQLite data is ignored if a user switches to MySQL; if this is intentional, consider making the behavior explicit in configuration or code comments to avoid confusion about missing migrated data.

## Individual Comments

### Comment 1
<location> `astrbot/core/db/mysql.py:103-106` </location>
<code_context>
+        async with self.AsyncSessionLocal() as session:
+            yield session
+
+    async def initialize(self) -> None:
+        """Initialize the database by creating tables if they do not exist."""
+        async with self.engine.begin() as conn:
+            await conn.run_sync(SQLModel.metadata.create_all)
+            await conn.commit()
+
</code_context>

<issue_to_address>
**issue (bug_risk):** Avoid calling `conn.commit()` inside `engine.begin()`; the context manager already manages the transaction.

Within `async with self.engine.begin() as conn:`, SQLAlchemy opens a transaction and commits or rolls back on exit. The extra `await conn.commit()` is unnecessary and can cause errors in some configurations. Please rely on the context manager’s automatic commit instead.
</issue_to_address>

### Comment 2
<location> `astrbot/core/db/mysql.py:840` </location>
<code_context>
+            result = await session.execute(query)
+            return list(result.scalars().all())
+
+    async def update_platform_session(
+        self,
+        session_id: str,
+        display_name: str | None = None,
+    ) -> None:
+        """Update a Platform session's updated_at timestamp and optionally display_name."""
+        async with self.get_db() as session:
+            session: AsyncSession
+            async with session.begin():
+                values: dict[str, T.Any] = {"updated_at": datetime.now(timezone.utc)}
+                if display_name is not None:
+                    values["display_name"] = display_name
+
+                await session.execute(
+                    update(PlatformSession)
+                    .where(col(PlatformSession.session_id) == session_id)
</code_context>

<issue_to_address>
**suggestion (bug_risk):** Align timestamp timezone handling for `updated_at` with the rest of the model to avoid mixed aware/naive datetimes.

Other timestamps in this module (e.g., stats and message history) use naive `datetime.now()`, so this is the only UTC-aware field. Depending on how `PlatformSession.updated_at` is defined, this mismatch can cause warnings or comparison issues. Please choose a single convention (UTC-aware or naive) and apply it consistently across the DB layer.

```suggestion
                values: dict[str, T.Any] = {"updated_at": datetime.now()}
```
</issue_to_address>

### Comment 3
<location> `astrbot/core/db/migration/migra_webchat_session.py:41` </location>
<code_context>
                 select(
                     col(PlatformMessageHistory.user_id),
-                    col(PlatformMessageHistory.sender_name),
+                    func.max(PlatformMessageHistory.sender_name).label("sender_name"),
                     func.min(PlatformMessageHistory.created_at).label("earliest"),
                     func.max(PlatformMessageHistory.updated_at).label("latest"),
</code_context>

<issue_to_address>
**question (bug_risk):** Aggregating `sender_name` with `MAX` fixes SQL grouping but produces an arbitrary name; confirm this matches the intended semantics.

`func.max` satisfies `ONLY_FULL_GROUP_BY` but picks the lexicographically greatest name, which is essentially arbitrary within the group. If the chosen `sender_name` has semantic or UI significance, consider deriving it from a specific row (e.g., earliest or latest message) rather than relying on `MAX`.
</issue_to_address>