fix: 修复Alembic元数据漂移检测

- 新增 021_add_billing_records_table.py 迁移（billing_records 表缺失）
- 更新 docs/schema-metadata-snapshot.json 同步最新表结构
- 7 张新表 + edit_templates 变更现已全部有对应 Alembic 迁移

alembic/versions/021_add_billing_records_table.py

@@ -0,0 +1,43 @@
+"""Task 3.09: Create billing_records table
+
+Revision ID: 021
+Revises: 020
+Create Date: 2026-07-03
+
+新增 billing_records 表，用于存储账单记录。
+"""
+
+import sqlalchemy as sa
+
+from alembic import op
+
+revision = "021"
+down_revision = "020"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "billing_records",
+        sa.Column("id", sa.String(36), primary_key=True),
+        sa.Column("user_id", sa.String(36), nullable=False, index=True),
+        sa.Column("plan_name", sa.String(50), nullable=False),
+        sa.Column("amount", sa.Float, nullable=False),
+        sa.Column("billing_cycle", sa.String(20), nullable=False),
+        sa.Column("status", sa.String(20), nullable=False, server_default="pending"),
+        sa.Column("payment_method", sa.String(50), nullable=True),
+        sa.Column("payment_id", sa.String(100), nullable=True),
+        sa.Column("invoice_url", sa.String(500), nullable=True),
+        sa.Column(
+            "created_at",
+            sa.DateTime(),
+            nullable=False,
+            server_default=sa.func.now(),
+        ),
+        sa.Column("paid_at", sa.DateTime(), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("billing_records")