feat: route background ai workloads by model

ai_knowledge_base_v2/apps/backend/alembic/versions/0022_model_routing.py

@@ -0,0 +1,52 @@
+"""add multi-model routing support
+
+Revision ID: 0022_model_routing
+Revises: 0021_periodic_report_async_jobs
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = "0022_model_routing"
+down_revision = "0021_periodic_report_async_jobs"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "sys_model",
+        sa.Column("is_default", sa.Integer(), nullable=False, server_default="0"),
+    )
+    op.create_index(
+        "ix_sys_model_available_default",
+        "sys_model",
+        ["enabled", "is_default", "id"],
+        unique=False,
+    )
+    # 兼容旧数据：原来唯一启用的模型直接成为默认主模型。
+    op.execute(
+        """
+        UPDATE sys_model
+        SET is_default = 1
+        WHERE enabled = 1
+          AND id = (
+              SELECT selected.id
+              FROM (
+                  SELECT id
+                  FROM sys_model
+                  WHERE enabled = 1
+                  ORDER BY id DESC
+                  LIMIT 1
+              ) AS selected
+          )
+        """
+    )
+
+
+def downgrade() -> None:
+    # 回滚到旧语义前只保留默认模型为启用，避免旧代码随机选到分流模型。
+    op.execute("UPDATE sys_model SET enabled = CASE WHEN is_default = 1 THEN 1 ELSE 0 END")
+    op.drop_index("ix_sys_model_available_default", table_name="sys_model")
+    op.drop_column("sys_model", "is_default")