Fix 6 issues surfaced by adversarial agent team (round 35)

Concurrency/lifecycle (P1, highest-value):
- singleton_resources.go: RegisterBackgroundTask now REPLACES a stopped
  task instead of reusing it. BackgroundTask.Start is sync.Once-guarded,
  so a task stopped on instance close and re-registered (Traefik
  close+recreate for the same provider) was permanently dead: IsTaskRunning
  returned false (started && !stopped) so callers retried a start that
  was a no-op. Metadata refresh, token cleanup, and health checks now
  resume across a recreate cycle.
- error_recovery.go: the shared graceful-degradation health task was
  bound to the FIRST instance's performHealthChecks (later instances'
  checks never ran) and any instance's Close stopped it for all. Added a
  global live-instance registry; the shared task runs every instance's
  checks and is stopped only when the last instance closes (mirrors the
  singleton-token-cleanup gate).

Storage/cache:
- hybrid.go recordL2Error: Store(now) then immediately compared
  time.Since(t)<1s against its own write => ALWAYS fired, so a single
  transient L2 error flipped the whole cache into fallback (full L2
  bypass -> upstream stampede). Now enters fallback only on a PRIOR
  error within the window (true "multiple errors"). Updated two tests
  that had encoded the old single-error-fallback bug.
- redis_pool.go: ConnectionPool.Put checked closed but sent on
  p.connections without holding p.mu; Close closed it under p.mu.
  Window => 'send on closed channel' panic at shutdown. Put now holds
  p.mu (the lock only Close used).

HTTP/transport:
- jwk.go fetchJWKS: added an explicit >1 MiB guard (was silent partial
  read, could yield a truncated keyset and spurious missing-kid).
- http_client_pool.go configKey: now includes every transport-behavior
  field it actually consumes (compression, HTTP2, keep-alives, timeouts,
  buffer sizes, dial) so behaviorally different configs no longer share
  one transport built with the first config's settings.

Each fix has a regression test. Verified: full source-order suite green,
golangci-lint 0 issues, 3x full -race -count=1 -shuffle=on ./... all exit 0.

internal/cache/backends/redis_pool_test.go

@@ -688,3 +688,47 @@ func TestRedisConn_RejectOversizedArgumentBytes(t *testing.T) {
 	require.Error(t, err)
 	assert.Contains(t, err.Error(), "arguments too large")
 }
+
+// TestConnectionPool_PutDuringClose_NoPanic hammers Put/Get concurrently with
+// Close. Before the fix, Put checked closed then sent on p.connections
+// without holding p.mu; if Close closed the channel in between, the send
+// panicked ("send on closed channel") at shutdown. Run under -race.
+func TestConnectionPool_PutDuringClose_NoPanic(t *testing.T) {
+	mr := NewMiniredisServer(t)
+
+	config := &PoolConfig{
+		Address:        mr.GetAddr(),
+		MaxConnections: 5,
+		ConnectTimeout: 5 * time.Second,
+		ReadTimeout:    3 * time.Second,
+		WriteTimeout:   3 * time.Second,
+	}
+
+	pool, err := NewConnectionPool(config)
+	require.NoError(t, err)
+
+	ctx := context.Background()
+	stop := make(chan struct{})
+	var wg sync.WaitGroup
+	for g := 0; g < 8; g++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for {
+				select {
+				case <-stop:
+					return
+				default:
+				}
+				if conn, err := pool.Get(ctx); err == nil {
+					pool.Put(conn) // may race Close
+				}
+			}
+		}()
+	}
+
+	time.Sleep(5 * time.Millisecond)
+	_ = pool.Close()
+	close(stop)
+	wg.Wait()
+}