fix: Extend sys_logs user_agent column length to support DingTalk agent (#1170)

The DingTalk agent sends user_agent strings that exceed the previous
255-character limit, causing log entries to be silently dropped.
Increase the column to VARCHAR(500).

backend/alembic/versions/068_alter_sys_logs_user_agent_length.py

@@ -0,0 +1,29 @@
+"""068_alter_sys_logs_user_agent_length
+
+Revision ID: a1b2c3d4e5f6
+Revises: e51127e9aa4a
+Create Date: 2026-06-09 00:00:00.000000
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision = 'a1b2c3d4e5f6'
+down_revision = 'e51127e9aa4a'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.alter_column('sys_logs', 'user_agent',
+                    existing_type=sa.VARCHAR(length=255),
+                    type_=sa.VARCHAR(length=500),
+                    existing_nullable=True)
+
+
+def downgrade():
+    op.alter_column('sys_logs', 'user_agent',
+                    existing_type=sa.VARCHAR(length=500),
+                    type_=sa.VARCHAR(length=255),
+                    existing_nullable=True)