perf(proxy): trim per-request hot-path work and db wait

- Guard debug body serialization with `log::log_enabled!`; previously
  serialized the filtered body to a throwaway String on every forward,
  even with debug logging off.
- Skip SSE parse + UTF-8 buffer loop when no usage collector and debug
  is off; the per-chunk `serde_json::from_str::<Value>` ran even in
  pure passthrough mode.
- Add cheap per-app SSE event pre-filter (string `contains`) so usage
  collectors only parse events that could contain usage (e.g. Claude
  `message_start` / `message_delta`).
- Skip non-streaming response body JSON parse when usage logging is
  disabled.
- Move `ProviderRouter::record_result` off the success response path
  via `tokio::spawn` for non-HalfOpen state; that call internally does
  `get_proxy_config_for_app` + `update_provider_health`, two SQLite
  ops that previously blocked TTFB.

Also: dedupe `usage_logging_enabled` (was duplicated in handlers.rs)
and merge `SseUsageCollector::{new, new_filtered}` into a single
constructor that takes `Option<StreamUsageEventFilter>`.

src-tauri/src/proxy/handler_config.rs

@@ -14,6 +14,12 @@ pub type ResponseUsageParser = fn(&Value) -> Option<TokenUsage>;
 /// 参数: (流式事件列表, 请求中的模型名称) -> 最终使用的模型名称
 pub type StreamModelExtractor = fn(&[Value], &str) -> String;
 
+/// 流式 usage 事件预过滤器类型别名。
+///
+/// 参数是 SSE `data:` 原始字符串。返回 false 时跳过 JSON parse，避免在
+/// token/chunk 高频路径上解析与 usage 无关的事件。
+pub type StreamUsageEventFilter = fn(&str) -> bool;
+
 /// 各 API 的使用量解析配置
 #[derive(Clone, Copy)]
 pub struct UsageParserConfig {
@@ -23,10 +29,32 @@ pub struct UsageParserConfig {
     pub response_parser: ResponseUsageParser,
     /// 流式响应中的模型提取器
     pub model_extractor: StreamModelExtractor,
+    /// 流式 usage 事件预过滤器
+    pub stream_event_filter: Option<StreamUsageEventFilter>,
     /// 应用类型字符串（用于日志记录）
     pub app_type_str: &'static str,
 }
 
+// ============================================================================
+// 流式 usage 事件预过滤
+// ============================================================================
+
+pub fn claude_stream_usage_event_filter(data: &str) -> bool {
+    data.contains("\"message_start\"") || data.contains("\"message_delta\"")
+}
+
+fn openai_stream_usage_event_filter(data: &str) -> bool {
+    data.contains("\"usage\"")
+}
+
+fn codex_stream_usage_event_filter(data: &str) -> bool {
+    data.contains("\"response.completed\"") || data.contains("\"usage\"")
+}
+
+fn gemini_stream_usage_event_filter(data: &str) -> bool {
+    data.contains("\"usageMetadata\"")
+}
+
 // ============================================================================
 // 模型提取器实现
 // ============================================================================
@@ -104,6 +132,7 @@ pub const CLAUDE_PARSER_CONFIG: UsageParserConfig = UsageParserConfig {
     stream_parser: TokenUsage::from_claude_stream_events,
     response_parser: TokenUsage::from_claude_response,
     model_extractor: claude_model_extractor,
+    stream_event_filter: Some(claude_stream_usage_event_filter),
     app_type_str: "claude",
 };
 
@@ -112,6 +141,7 @@ pub const OPENAI_PARSER_CONFIG: UsageParserConfig = UsageParserConfig {
     stream_parser: TokenUsage::from_openai_stream_events,
     response_parser: TokenUsage::from_openai_response,
     model_extractor: openai_model_extractor,
+    stream_event_filter: Some(openai_stream_usage_event_filter),
     app_type_str: "codex",
 };
 
@@ -120,6 +150,7 @@ pub const CODEX_PARSER_CONFIG: UsageParserConfig = UsageParserConfig {
     stream_parser: TokenUsage::from_codex_stream_events_auto,
     response_parser: TokenUsage::from_codex_response_auto,
     model_extractor: codex_auto_model_extractor,
+    stream_event_filter: Some(codex_stream_usage_event_filter),
     app_type_str: "codex",
 };
 
@@ -128,6 +159,7 @@ pub const GEMINI_PARSER_CONFIG: UsageParserConfig = UsageParserConfig {
     stream_parser: TokenUsage::from_gemini_stream_chunks,
     response_parser: TokenUsage::from_gemini_response,
     model_extractor: gemini_model_extractor,
+    stream_event_filter: Some(gemini_stream_usage_event_filter),
     app_type_str: "gemini",
 };