feat(flywheel): few-shot 在线检索注入打通风险规则编译链路

- 新增 FewShotStore：独立 Qdrant collection few_shot_samples，向量 upsert/search/delete，
  全程失败降级不阻塞主链路
- 新增 FewShotIngestionService：RiskObservation confirmed/false_positive → FewShotSample +
  向量，带 sample_key 幂等去重
- 新增 FewShotRetriever：按 case 特征检索相似历史样本，去重 + token 预算 + 单条字符上限裁剪
- risk_observations.create_feedback commit 后挂 hook 自动入库，带 feature flag 和 try/except 兜底
- risk_rule_generation_prompt 新增 few_shot_samples 可选参数，合并进 examples 并标
  source=historical_confirmed；risk_rule_generation 构造 prompt 前调 retriever，失败降级为空

server/src/app/services/risk_rule_generation.py

@@ -234,6 +234,10 @@ class RiskRuleGenerationService:
             }
             for item in fields
         ]
+        few_shot_samples = self._retrieve_few_shot_samples(
+            domain=domain,
+            natural_language=natural_language,
+        )
         messages = build_risk_rule_compiler_messages(
             domain=domain,
             domain_label=BUSINESS_DOMAIN_LABELS[domain],
@@ -243,6 +247,7 @@ class RiskRuleGenerationService:
             expense_category_label=expense_category_label,
             natural_language=natural_language,
             available_fields=field_payload,
+            few_shot_samples=few_shot_samples,
         )
         answer = self.runtime_chat_service.complete(
             messages,
@@ -263,6 +268,29 @@ class RiskRuleGenerationService:
         payload = unwrap_semantic_plan_payload(payload)
         return self._sanitize_model_draft(payload, fields=fields)
 
+    def _retrieve_few_shot_samples(
+        self,
+        *,
+        domain: str,
+        natural_language: str,
+    ) -> list[dict[str, Any]]:
+        """检索已确认历史样本，失败降级为空列表。"""
+
+        import os
+
+        if os.environ.get("FEW_SHOT_INJECTION_ENABLED", "true").strip().lower() in {"0", "false", "no"}:
+            return []
+        try:
+            from app.services.few_shot_retrieval import FewShotRetriever
+
+            retriever = FewShotRetriever.from_session(self.db)
+            return retriever.retrieve_for_risk_rule_generation(
+                domain=domain,
+                natural_language=natural_language,
+            )
+        except Exception:
+            return []
+
     def _sanitize_model_draft(
         self,
         payload: dict[str, Any],