feat(quota): implement unified quota management system and add community free plan

- Add `default_free_plan.py` to define the configuration for the Community Free Plan.
- Refactor `quota_stub.py` as a unified entry point, delegating checks to `core/quota_manager`.
- Implement core logic in `quota_manager.py` to support retrieving quotas from the premium module or configuration files.
- Update `tenant_subscription_controller` to return Community Free Plan information.

api/app/config/default_free_plan.py

@@ -0,0 +1,30 @@
+"""
+社区版默认免费套餐配置
+当无法从 SaaS 版获取 premium 模块时，使用此配置作为兜底
+"""
+
+DEFAULT_FREE_PLAN = {
+    "name": "记忆体验版",
+    "category": "saas_personal",
+    "tier_level": 0,
+    "version": "1.0",
+    "status": True,
+    "price": 0,
+    "billing_cycle": "permanent_free",
+    "core_value": "感受永久记忆",
+    "tech_support": "社群交流",
+    "sla_compliance": "无",
+    "page_customization": "无",
+    "theme_color": "#64748B",
+    "quotas": {
+        "workspace_quota": 1,
+        "skill_quota": 5,
+        "app_quota": 2,
+        "knowledge_capacity_quota": 0.3,
+        "memory_engine_quota": 1,
+        "end_user_quota": 1,
+        "ontology_project_quota": 3,
+        "model_quota": 1,
+        "api_ops_rate_limit": 50,
+    },
+}