Fix security regression: use localhost-only fallback instead of permissive mode

Codex independent review identified a critical security issue: when cluster
validation fails, the previous fix fell back to permissive mode (allowing
ALL nodes), making the proxy a potential SSRF/network scanner for any
container that could reach the socket.

NEW BEHAVIOR:
When cluster validation is unavailable (IPC blocked), fall back to
localhost-only validation instead of permissive mode. This maintains
security while still allowing self-monitoring.

Implementation:
- Added validateAsLocalhost() method to nodeValidator
- Calls discoverLocalHostAddresses() to get local IPs/hostnames
- Only allows requests matching the local host
- Blocks requests to other cluster members or arbitrary hosts

Test results on delly (clustered node with IPC blocked):
- Request to 192.168.0.5 (self): ALLOWED, temps fetched
- Request to 192.168.0.134 (cluster peer): BLOCKED with node_not_localhost
- No more "allowing all nodes" security regression

Related to #571 - addresses Codex security audit feedback

This prevents the proxy from being abused as a network scanner while
still solving the original temperature monitoring issue.

cmd/pulse-sensor-proxy/validation.go

@@ -358,13 +358,14 @@ func (v *nodeValidator) Validate(ctx context.Context, node string) error {
 		allowed, err := v.matchesCluster(ctx, node)
 		if err != nil {
 			// Cluster query failed (e.g., IPC permission denied, running in LXC)
-			// Fall through to permissive mode rather than blocking all requests
+			// Fall back to localhost-only validation instead of permissive mode
 			v.recordFailure(validationReasonClusterFailed)
 			log.Warn().
 				Err(err).
 				Str("node", node).
-				Msg("SECURITY: Cluster validation unavailable - allowing all nodes. Configure allowed_nodes in config to restrict access.")
+				Msg("SECURITY: Cluster validation unavailable - falling back to localhost-only validation. Configure allowed_nodes for cluster-wide access.")
-			// Fall through to permissive mode below
+			// Attempt to validate against localhost addresses
+			return v.validateAsLocalhost(ctx, node)
 		} else if !allowed {
 			return v.deny(node, validationReasonNotClusterMember)
 		} else {
@@ -379,6 +380,41 @@ func (v *nodeValidator) Validate(ctx context.Context, node string) error {
 	return nil
 }
 
+func (v *nodeValidator) validateAsLocalhost(ctx context.Context, node string) error {
+	// When cluster validation is unavailable, only allow access to localhost
+	// This maintains security while allowing self-monitoring
+	if ctx == nil {
+		ctx = context.Background()
+	}
+
+	// Try to discover local host addresses
+	localAddrs, err := discoverLocalHostAddresses()
+	if err != nil {
+		log.Warn().Err(err).Msg("Failed to discover local host addresses for fallback validation")
+		// If we can't even discover localhost, deny access
+		return v.deny(node, validationReasonClusterFailed)
+	}
+
+	// Check if the requested node matches any local address
+	normalized := normalizeAllowlistEntry(node)
+	if normalized == "" {
+		normalized = strings.ToLower(strings.TrimSpace(node))
+	}
+
+	for _, localAddr := range localAddrs {
+		if strings.EqualFold(localAddr, normalized) {
+			log.Debug().
+				Str("node", node).
+				Str("matched_local", localAddr).
+				Msg("Node validated as localhost (cluster validation unavailable)")
+			return nil
+		}
+	}
+
+	// Node doesn't match any local address - deny
+	return v.deny(node, "node_not_localhost")
+}
+
 func (v *nodeValidator) matchesAllowlist(ctx context.Context, node string) (bool, error) {
 	normalized := normalizeAllowlistEntry(node)
 	if normalized != "" {