feat(flywheel): 抽公共 EmbeddingProvider 并新增 FewShotSample 模型

- 从 knowledge_rag_runtime 抽出 embedding 调用逻辑为独立 EmbeddingProvider，
  复用现有 HTTP 纯函数，RAG 路径零回归
- 新增 FewShotSample 表模型（样本池），注册到 db/base.py 和 models/__init__.py
  供 few-shot 飞轮沉淀已确认风险观测

server/src/app/models/__init__.py

@@ -8,6 +8,7 @@ from app.models.budget import BudgetAllocation, BudgetReservation, BudgetTransac
 from app.models.employee_change_log import EmployeeChangeLog
 from app.models.employee_behavior_profile import EmployeeBehaviorProfileSnapshot
 from app.models.employee import Employee
+from app.models.few_shot_sample import FewShotSample
 from app.models.financial_record import (
     AccountsPayableRecord,
     AccountsReceivableRecord,
@@ -49,6 +50,7 @@ __all__ = [
     "EmployeeChangeLog",
     "ExpenseClaim",
     "ExpenseClaimItem",
+    "FewShotSample",
     "HermesTaskConfig",
     "HermesTaskExecutionLog",
     "HermesRiskReport",