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/hybrid.go

@@ -663,18 +663,22 @@ func (h *HybridBackend) healthMonitor() {
 
 // recordL2Error records the timestamp of an L2 error
 func (h *HybridBackend) recordL2Error() {
-	h.lastL2Error.Store(time.Now())
+	now := time.Now()
 
-	// Check if we should enter fallback mode based on recent errors
+	// Enter fallback only if there was a PRIOR error within the last second —
+	// i.e. this is at least the second L2 error in quick succession.
+	// Checking a freshly-written timestamp would always be "recent" and
+	// enter fallback on a single transient L2 error (cache stampede).
 	if !h.fallbackMode.Load() {
-		// Simple heuristic: if we've had an error in the last second, consider L2 unhealthy
 		if lastErr := h.lastL2Error.Load(); lastErr != nil {
-			if t, ok := lastErr.(time.Time); ok && time.Since(t) < time.Second {
+			if t, ok := lastErr.(time.Time); ok && now.Sub(t) < time.Second {
 				h.fallbackMode.Store(true)
 				h.logger.Warnf("Multiple L2 errors detected, entering fallback mode")
 			}
 		}
 	}
+
+	h.lastL2Error.Store(now)
 }
 
 // extractCacheType attempts to determine the cache type from the key