feat(ai): 扩展 AiConfig 支持多 Provider 配置

- config/default.toml 新增 providers 子段（claude/openai/ollama）
- erp-server/config.rs AiConfig 新增 quota_check_enabled + providers HashMap
- erp-ai/config.rs 新增 ProviderType 枚举 + ProviderConfig 结构体

crates/erp-server/src/config.rs

@@ -99,6 +99,26 @@ pub struct AiConfig {
     pub temperature: f32,
     pub cache_ttl_seconds: u64,
     pub rate_limit_patient_daily: u32,
+    #[serde(default)]
+    pub quota_check_enabled: bool,
+    #[serde(default)]
+    pub providers: std::collections::HashMap<String, ProviderConfig>,
+}
+
+#[derive(Debug, Clone, Deserialize)]
+pub struct ProviderConfig {
+    pub provider_type: String,
+    pub api_key_env: Option<String>,
+    pub base_url: Option<String>,
+    pub default_model: String,
+    pub max_tokens: u32,
+    pub temperature: f32,
+    #[serde(default = "default_true")]
+    pub is_enabled: bool,
+}
+
+fn default_true() -> bool {
+    true
 }
 
 #[derive(Debug, Clone, Deserialize)]