feat: squash merge all changes from develop-0.1.75

Squash of 124 commits from the legacy develop branch (develop-0.1.75)
onto a clean v0.1.75 upstream base, to simplify future upstream merges.

Key changes included:
- Refactor scope-level rate limiting to model-level rate limiting
- Antigravity gateway service improvements (smart retry, error policy)
- Digest session store (flat cache replacing Trie-based store)
- Client disconnect detection during streaming
- Gemini messages compatibility service enhancements
- Scheduler shuffle for thundering herd prevention
- Session hash generation improvements
- Frontend customizations (WeChat service, HomeView, etc.)
- Ops monitoring scope cleanup

backend/internal/service/gateway_service_benchmark_test.go

@@ -14,7 +14,7 @@ func BenchmarkGenerateSessionHash_Metadata(b *testing.B) {
 
 	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {
-		parsed, err := ParseGatewayRequest(body)
+		parsed, err := ParseGatewayRequest(body, "")
 		if err != nil {
 			b.Fatalf("解析请求失败: %v", err)
 		}