feat(workspace): extend Workspace entity with subscription fields

- Add subscription_plan (free/pro/enterprise) with free as default
- Add subscription_status (active/cancelled/expired)
- Add subscription_expires_at for expiration tracking
- Add max_projects quota (free: 3, pro/enterprise: unlimited)
- Add max_storage_gb and used_storage_gb for storage tracking
- Backward compatible with existing code (all new fields have defaults)

Phase 4 Task 9/68 completed

packages/domain/entities.py

@@ -41,6 +41,14 @@ class Workspace:
     id: str
     name: str
     owner_user_id: str
+    # 订阅相关字段
+    subscription_plan: str = "free"  # free, pro, enterprise
+    subscription_status: str = "active"  # active, cancelled, expired
+    subscription_expires_at: datetime | None = None
+    # 配额限制
+    max_projects: int = 3  # free: 3, pro: unlimited, enterprise: unlimited
+    max_storage_gb: int = 10  # free: 10, pro: 100, enterprise: 1000
+    used_storage_gb: float = 0.0
     created_at: datetime = field(default_factory=lambda: datetime.now(timezone.utc))