feat(server): 设置持久化新增 LLM 模型表与主题字段

- SettingsLlmForm 新增 models 列表（SettingsModelRow：slot/provider/url/apiKey/modelId/type），支持多模型行持久化
- settings 服务读写模型表与主题相关字段，更新 test_settings_persistence 测试

server/src/app/schemas/settings.py

@@ -56,6 +56,23 @@ class SettingsSessionForm(BaseModel):
     conversationRetentionDays: int = Field(default=3, ge=1, le=10)
 
 
+class SettingsModelRow(BaseModel):
+    slot: str = Field(min_length=1, max_length=64)
+    provider: str = Field(min_length=1, max_length=64)
+    url: str = Field(min_length=1, max_length=512)
+    apiKey: str = Field(default="", max_length=1024)
+    apiKeyConfigured: bool = False
+    modelId: str = Field(min_length=1, max_length=255)
+    type: Literal["llm", "embedding", "rerank"] = "llm"
+
+    @field_validator("slot", "provider", "url", "apiKey", "modelId", mode="before")
+    @classmethod
+    def strip_model_row_string(cls, value: str | None) -> str | None:
+        if value is None:
+            return None
+        return value.strip()
+
+
 class SettingsLlmForm(BaseModel):
     mainProvider: str = Field(min_length=1, max_length=64)
     mainModel: str = Field(min_length=1, max_length=255)
@@ -80,6 +97,7 @@ class SettingsLlmForm(BaseModel):
     rerankerEndpoint: str = Field(min_length=1, max_length=512)
     rerankerApiKey: str = Field(default="", max_length=1024)
     rerankerApiKeyConfigured: bool = False
+    models: list[SettingsModelRow] = Field(default_factory=list)
 
     @field_validator(
         "mainProvider",
@@ -201,7 +219,7 @@ class ModelConnectivityTestRequest(BaseModel):
     model: str = Field(min_length=1, max_length=255)
     api_key: str | None = Field(default=None, max_length=1024)
     capability: Literal["chat", "embedding", "reranker"] = "chat"
-    slot: Literal["main", "backup", "embedding", "reranker"] | None = None
+    slot: str | None = Field(default=None, max_length=64)
 
     @field_validator("provider", "endpoint", "model", "api_key", mode="before")
     @classmethod
@@ -234,7 +252,7 @@ class SettingsCacheClearRead(BaseModel):
 
 
 class RuntimeModelConfigRead(BaseModel):
-    slot: Literal["main", "backup", "embedding", "reranker"]
+    slot: str
     provider: str
     model: str
     endpoint: str