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/services/embedding_provider.py

@@ -0,0 +1,138 @@
+"""公共 Embedding 提供者。
+
+把 ``knowledge_rag_runtime`` 里 embedding 调用逻辑抽出来，供 RAG 和
+few-shot 检索复用。本模块只依赖现有模块级纯函数和 ``RuntimeModelConfig``，
+不改动 ``_LightRagRuntime`` 的行为，RAG 路径保持零回归风险。
+
+典型用法::
+
+    provider = EmbeddingProvider.from_settings(session)
+    vectors = provider.embed(["差旅超标", "票单不一致"])
+    dim = provider.dimension()
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from app.core.logging import get_logger
+from app.services.knowledge_rag_runtime import (
+    DEFAULT_EMBEDDING_TIMEOUT_SECONDS,
+    KnowledgeRagError,
+    RuntimeModelConfig,
+    _build_headers,
+    _ensure_path,
+    _extract_embedding_vectors,
+    _normalize_endpoint,
+    _send_json_request,
+)
+
+logger = get_logger("app.services.embedding_provider")
+
+
+def _runtime_model_config_from_dict(config: dict[str, str]) -> RuntimeModelConfig:
+    """把 SettingsService.get_runtime_model_config 返回的 dict 转成 dataclass。"""
+
+    return RuntimeModelConfig(
+        slot=str(config.get("slot") or "embedding"),
+        provider=str(config.get("provider") or ""),
+        model=str(config.get("model") or ""),
+        endpoint=str(config.get("endpoint") or ""),
+        api_key=str(config.get("apiKey") or ""),
+        capability=str(config.get("capability") or ""),
+    )
+
+
+class EmbeddingProvider:
+    """对 embedding 模型的轻量封装。
+
+    设计要点：
+    - 持有一个 ``RuntimeModelConfig``，构造即固定，不依赖 LightRAG。
+    - 复用 ``knowledge_rag_runtime`` 的 HTTP 调用纯函数，行为与 RAG 完全一致。
+    - 维度采用惰性探测（首次 embed 后缓存），避免空构造就打远端。
+    """
+
+    def __init__(self, config: RuntimeModelConfig) -> None:
+        self.config = config
+        self._dimension: int | None = None
+
+    @classmethod
+    def from_settings(cls, session: Any) -> "EmbeddingProvider":
+        """从 SettingsService 取 embedding 配置构造 provider。"""
+
+        from app.services.settings import SettingsService
+
+        raw = SettingsService(session).get_runtime_model_config("embedding")
+        return cls(_runtime_model_config_from_dict(raw))
+
+    def embed(self, texts: list[str]) -> list[list[float]]:
+        """对一组文本做 embedding，返回与输入等长的向量列表。"""
+
+        if not texts:
+            return []
+        return _request_embeddings_public(self.config, texts)
+
+    def dimension(self) -> int:
+        """探测 embedding 维度，结果缓存。失败抛 KnowledgeRagError。"""
+
+        if self._dimension is None:
+            vectors = self.embed(["dimension probe"])
+            if not vectors or not isinstance(vectors[0], list):
+                raise KnowledgeRagError("无法从 embedding 模型返回结果中解析向量维度。")
+            self._dimension = len(vectors[0])
+            if self._dimension <= 0:
+                raise KnowledgeRagError("embedding 模型返回了无效的向量维度。")
+        return self._dimension
+
+
+def _request_embeddings_public(
+    config: RuntimeModelConfig,
+    texts: list[str],
+) -> list[list[float]]:
+    """按 provider 分支构造 embedding 请求。
+
+    与 ``_LightRagRuntime._request_embeddings`` 实现保持一致，
+    保证 few-shot 检索与 RAG 走同一套调用语义。
+    """
+
+    from app.services.model_connectivity import AZURE_API_VERSION
+
+    if config.provider == "Azure OpenAI":
+        from app.services.knowledge_rag_runtime import _build_azure_deployment_base
+
+        url = f"{_build_azure_deployment_base(config.endpoint, config.model)}/embeddings?api-version={AZURE_API_VERSION}"
+        payload: dict[str, Any] = {"input": texts}
+        status_code, body = _send_json_request(
+            "POST",
+            url,
+            headers=_build_headers(config.api_key, use_bearer=False, use_api_key=True),
+            payload=payload,
+            timeout_seconds=DEFAULT_EMBEDDING_TIMEOUT_SECONDS,
+        )
+    elif config.provider == "Ollama":
+        url = _ensure_path(_normalize_endpoint(config.endpoint), "api/embed")
+        payload = {"model": config.model, "input": texts}
+        status_code, body = _send_json_request(
+            "POST",
+            url,
+            headers={"Content-Type": "application/json", "Accept": "application/json"},
+            payload=payload,
+            timeout_seconds=DEFAULT_EMBEDDING_TIMEOUT_SECONDS,
+        )
+    else:
+        url = _ensure_path(_normalize_endpoint(config.endpoint), "embeddings")
+        payload = {"model": config.model, "input": texts}
+        status_code, body = _send_json_request(
+            "POST",
+            url,
+            headers=_build_headers(config.api_key, use_bearer=True),
+            payload=payload,
+            timeout_seconds=DEFAULT_EMBEDDING_TIMEOUT_SECONDS,
+        )
+
+    from http import HTTPStatus
+
+    if status_code >= HTTPStatus.BAD_REQUEST:
+        raise KnowledgeRagError(f"embedding 模型返回异常状态码 {status_code}。")
+
+    return _extract_embedding_vectors(body, provider=config.provider)