fix: compare platform message history cutoff in UTC

platform_message_history.created_at stores UTC wall-clock values
(written via TimestampMixin's datetime.now(timezone.utc)), but
delete_platform_message_offset computed the cutoff from naive local
datetime.now(). On hosts outside UTC the comparison was shifted by the
local offset, so the wrong slice of history was deleted.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

astrbot/core/db/sqlite.py

@@ -851,7 +851,8 @@ class SQLiteDatabase(BaseDatabase):
         async with self.get_db() as session:
             session: AsyncSession
             async with session.begin():
-                now = datetime.now()
+                # created_at stores UTC wall-clock values, so compare in UTC.
+                now = datetime.now(timezone.utc)
                 cutoff_time = now - timedelta(seconds=offset_sec)
                 await session.execute(
                     delete(PlatformMessageHistory).where(

tests/unit/test_datetime_storage.py

@@ -7,7 +7,13 @@ import pytest
 from sqlalchemy import Column, DateTime, MetaData, create_engine, literal, select
 from sqlmodel import SQLModel
 
-from astrbot.core.db.po import ConversationV2, PlatformStat
+import astrbot.core.db.sqlite as sqlite_module
+from astrbot.core.db.po import (
+    ConversationV2,
+    PlatformMessageHistory,
+    PlatformStat,
+)
+from astrbot.core.db.sqlite import SQLiteDatabase
 from astrbot.core.db.vec_db.faiss_impl.document_storage import BaseDocModel, Document
 from astrbot.core.knowledge_base.models import BaseKBModel, KnowledgeBase
 
@@ -95,3 +101,50 @@ def test_existing_datetime_rows_keep_naive_reads_and_filters(
             assert result.isoformat() == "2024-01-02T03:04:05.123456"
     finally:
         engine.dispose()
+
+
+@pytest.mark.asyncio
+async def test_delete_platform_message_offset_compares_in_utc(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+):
+    """The history cutoff must match the UTC wall time stored in created_at.
+
+    Args:
+        tmp_path: Temporary directory for the database file.
+        monkeypatch: Fixture used to simulate a host in a non-UTC timezone.
+    """
+    now_utc = datetime.now(timezone.utc)
+    inside_window = (now_utc - timedelta(hours=20)).replace(tzinfo=None)
+    outside_window = (now_utc - timedelta(hours=30)).replace(tzinfo=None)
+
+    db = SQLiteDatabase(str(tmp_path / "history.db"))
+    await db.initialize()
+    try:
+        async with db.get_db() as session:
+            async with session.begin():
+                for created_at in (inside_window, outside_window):
+                    session.add(
+                        PlatformMessageHistory(
+                            platform_id="p1",
+                            user_id="u1",
+                            content={},
+                            created_at=created_at,
+                        )
+                    )
+
+        class LocalTime(datetime):
+            """Simulate a host whose local timezone is UTC+8."""
+
+            @classmethod
+            def now(cls, tz=None):
+                if tz is None:
+                    return now_utc.replace(tzinfo=None) + timedelta(hours=8)
+                return now_utc.astimezone(tz)
+
+        monkeypatch.setattr(sqlite_module, "datetime", LocalTime)
+        await db.delete_platform_message_offset("p1", "u1", offset_sec=86400)
+
+        rows = await db.get_platform_message_history("p1", "u1")
+        assert [row.created_at for row in rows] == [outside_window]
+    finally:
+        await db.engine.dispose()