perf(network): mobile stability and data-usage fixes from network audit

Stability:
- transient timeouts no longer classified as ISP blocking; they fall
  through to retry backoff (hard blocks - DNS/RST/cert - still abort)
- connectivity change now closes idle Go connections in every network
  mode (debounced), so pooled sockets from the old interface are not
  reused after a wifi/cellular handoff
- download body reads get a 60s stall watchdog that cancels and
  surfaces a retryable network error instead of hanging; distinct
  from user cancellation
- ResponseHeaderTimeout 45s on all transports; IdleConnTimeout 90->60s;
  dial timeout 30->10s; retry backoff gains full jitter; Retry-After
  honored on 5xx and Deezer 429

Data usage / speed:
- cover downloads deduplicated with singleflight plus a 24MB/15min
  in-memory cache keyed by final URL (album batches fetched the same
  1800px cover once per track before)
- song.link availability cached (30min positive / 5min negative)
  in front of the 9-req/min rate limiter
- uTLS Cloudflare path now pools one HTTP/2 connection per host with
  a shared TLS session cache instead of a full handshake per request

Deezer artist track-count N+1 kept: counts feed the discography
download UI and are already amortized by the artist TTL cache.

go_backend/deezer.go

@@ -1269,6 +1269,10 @@ func (c *DeezerClient) getJSON(ctx context.Context, endpoint string, dst any) er
 	for attempt := 0; attempt <= deezerMaxRetries; attempt++ {
 		if attempt > 0 {
 			delay := deezerRetryDelay * time.Duration(1<<(attempt-1))
+			var apiErr *deezerAPIError
+			if errors.As(lastErr, &apiErr) && apiErr.RetryAfter > 0 {
+				delay = apiErr.RetryAfter
+			}
 			GoLog("[Deezer] Retry %d/%d after %v...\n", attempt, deezerMaxRetries, delay)
 			time.Sleep(delay)
 		}
@@ -1292,6 +1296,7 @@ func (c *DeezerClient) getJSON(ctx context.Context, endpoint string, dst any) er
 type deezerAPIError struct {
 	StatusCode int
 	Body       string
+	RetryAfter time.Duration
 }
 
 func (e *deezerAPIError) Error() string {
@@ -1329,7 +1334,7 @@ func (c *DeezerClient) doGetJSON(ctx context.Context, endpoint string, dst any)
 	}
 
 	if resp.StatusCode != http.StatusOK {
-		return &deezerAPIError{StatusCode: resp.StatusCode, Body: string(body)}
+		return &deezerAPIError{StatusCode: resp.StatusCode, Body: string(body), RetryAfter: getRetryAfterDuration(resp)}
 	}
 
 	return json.Unmarshal(body, dst)