fix(pipeline): escalate idle keepalive backoff to 20s

Previous cap of 2s caused ~1200 requests/5min idle with 15 deployments.
New escalation: 20ms→80ms→200ms→500ms→2s→5s→10s→20s.
After 15+ consecutive empties, sessions poll every 20s.

Estimated idle reduction: ~1200/5min → ~200/5min.
Zero latency impact on active traffic — select! races timer against
client reads, so real data fires immediately.

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

Author: yyoyoian-pixel

src/tunnel_client.rs

@@ -1638,8 +1638,10 @@ async fn tunnel_loop(
                 0 => Duration::from_millis(20),
                 1 => Duration::from_millis(80),
                 2 => Duration::from_millis(200),
-                3 => Duration::from_millis(500),
-                _ => Duration::from_secs(2),
+                3 => Duration::from_secs(2),
+                4..=7 => Duration::from_secs(5),
+                8..=14 => Duration::from_secs(10),
+                _ => Duration::from_secs(20),
             };
             if consecutive_empty > 0 {
                 // Wait for either the backoff timer or client data.