refactor: 删除未使用的 api_external_url 配置并完善 runtime 文档

- 删除 SupabaseSettings 中未使用的 api_external_url computed field
- 更新测试文件移除相关测试用例
- backend/AGENTS.md 新增软删除设计规则
- runtime-database.md 更新表结构（删除 user_agents，表名更新为 agent_chat_sessions/messages，system_agents）
- runtime-frontend.md 补充路由结构和功能模块说明
- 根 AGENTS.md 清理过时技能路径引用

backend/AGENTS.md

@@ -164,6 +164,26 @@ Use `schemas / repository / service` pattern:
 - service_role key is backend-only; never expose credentials
 - Prohibit calling Supabase Admin API (service_role key) from repository/service layers
 
+### Soft Delete
+
+**Soft delete marks data as invisible, not cascade delete.**
+
+- Use `deleted_at: datetime | None` column (via `SoftDeleteMixin`)
+- **Query filtering**: Repository `_apply_soft_delete_filter()` auto-excludes deleted records
+- **No automatic cascade**: Related data stays intact; visibility controlled by JOIN filtering
+- **Cascade only for strong dependencies**: When parent deletion must invalidate children, implement in Service layer explicitly
+- **Recovery**: Only restore the record itself; related data visibility restored automatically via queries
+- **Unique constraints**: Use partial indexes excluding `deleted_at IS NOT NULL` to allow re-creation
+
+```python
+# Partial unique index in migration
+op.execute("""
+    CREATE UNIQUE INDEX ux_user_email 
+    ON users(email) 
+    WHERE deleted_at IS NULL
+""")
+```
+
 ### Migrations
 
 - **Alembic is the single source of truth** for schema migrations