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

config/default.toml

@@ -48,6 +48,33 @@ max_tokens = 2048
 temperature = 0.3
 cache_ttl_seconds = 604800
 rate_limit_patient_daily = 10
+quota_check_enabled = true
+
+[ai.providers.claude]
+provider_type = "claude"
+api_key_env = "ANTHROPIC_API_KEY"
+base_url = "https://api.anthropic.com"
+default_model = "claude-sonnet-4-6"
+max_tokens = 2048
+temperature = 0.3
+is_enabled = true
+
+[ai.providers.openai]
+provider_type = "openai"
+api_key_env = "OPENAI_API_KEY"
+base_url = "https://api.openai.com"
+default_model = "gpt-4o"
+max_tokens = 2048
+temperature = 0.3
+is_enabled = false
+
+[ai.providers.ollama]
+provider_type = "ollama"
+base_url = "http://localhost:11434"
+default_model = "qwen2.5:7b"
+max_tokens = 2048
+temperature = 0.3
+is_enabled = false
 
 [storage]
 upload_dir = "./uploads"