fix(tunnel): 256KB upload cap, 10ms gap timeout, 8KB extend threshold

Reduces upload chunk size to prevent large video uploads from starving
heartbeat polls in shared batches. Adaptive accumulation:
- 50ms initial window, 10ms per-read gap timeout
- >= 8KB triggers extended 1s window (capped at 256KB)
- Smaller chunks clear batches faster, heartbeats get through

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: vahidlazio

src/tunnel_client.rs

@@ -1512,17 +1512,20 @@ async fn upload_task(
             if eof_seen.load(Ordering::Relaxed) { break; }
             let now = tokio::time::Instant::now();
             if now >= deadline { break; }
-            if data.len() >= 1024 * 1024 { break; } // 1MB cap
+            if data.len() >= 256 * 1024 { break; } // 256KB cap
             let remaining = deadline - now;
-            match tokio::time::timeout(remaining, reader.read(&mut buf)).await {
+            // Cap per-read wait at 10ms: if no data for 10ms (gap
+            // between video chunk and heartbeat), send what we have.
+            let read_timeout = Duration::from_millis(10).min(remaining);
+            match tokio::time::timeout(read_timeout, reader.read(&mut buf)).await {
                 Ok(Ok(0)) => {
                     upload_closed.store(true, Ordering::Release);
                     break;
                 }
                 Ok(Ok(more_n)) => {
                     data.extend_from_slice(&buf[..more_n]);
                     // Extend window if we hit 32KB threshold
-                    if !extended && data.len() >= 32 * 1024 {
+                    if !extended && data.len() >= 8 * 1024 {
                         deadline = tokio::time::Instant::now() + Duration::from_secs(1);
                         extended = true;
                     }