[platform] Add flux-plunger controller to fix HelmRelease errors (#1843)

## What this PR does

This PR introduces a new flux-plunger controller that automatically
fixes HelmRelease resources experiencing "has no deployed releases"
errors by cleaning up corrupted Helm release secrets.

### Problem

When Helm releases fail with "has no deployed releases" error, FluxCD
cannot recover automatically. This typically happens when:
- Helm release secrets are corrupted
- Failed upgrades leave the release in an inconsistent state
- Manual intervention is required to delete the problematic secret

### Solution

The flux-plunger controller:
1. Watches HelmRelease resources for "has no deployed releases" errors
2. Suspends the HelmRelease (with `flux-client-side-apply` field
manager)
3. Identifies and deletes the latest (problematic) Helm release secret
4. Tracks processed versions via annotation to prevent recursive
deletion
5. Unsuspends the HelmRelease to allow FluxCD to retry

**Key features:**
- Only deletes one secret at a time (prevents cascading deletions)
- Uses version tracking: if `latest+1 == processed`, skips (already
handled)
- Handles suspended HelmReleases: only unsuspends if it was suspended by
flux-plunger itself
- Gracefully handles invalid annotations by treating them as unprocessed

### Components

- **Controller:** `internal/controller/fluxplunger/flux_plunger.go`
- **Main:** `cmd/flux-plunger/main.go`  
- **Helm Chart:** `packages/system/flux-plunger/`
- **Platform Integration:** Added to `paas-full.yaml` bundle

### Release note

```release-note
[platform] Add flux-plunger controller to automatically fix HelmRelease resources with "has no deployed releases" error by cleaning up corrupted Helm secrets
```

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

## Release Notes

* **New Features**
* Introduced a new controller that automatically resolves HelmRelease
resource failures with built-in health probes and graceful recovery
procedures.
  * Enabled by default in system installations.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

cmd/flux-plunger/main.go

@@ -0,0 +1,151 @@
+/*
+Copyright 2025.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package main
+
+import (
+	"crypto/tls"
+	"flag"
+	"os"
+
+	// Import all Kubernetes client auth plugins (e.g. Azure, GCP, OIDC, etc.)
+	// to ensure that exec-entrypoint and run can make use of them.
+	_ "k8s.io/client-go/plugin/pkg/client/auth"
+
+	helmv2 "github.com/fluxcd/helm-controller/api/v2"
+	"k8s.io/apimachinery/pkg/runtime"
+	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
+	clientgoscheme "k8s.io/client-go/kubernetes/scheme"
+	ctrl "sigs.k8s.io/controller-runtime"
+	"sigs.k8s.io/controller-runtime/pkg/healthz"
+	"sigs.k8s.io/controller-runtime/pkg/log/zap"
+	metricsserver "sigs.k8s.io/controller-runtime/pkg/metrics/server"
+
+	"github.com/cozystack/cozystack/internal/controller/fluxplunger"
+	// +kubebuilder:scaffold:imports
+)
+
+var (
+	scheme   = runtime.NewScheme()
+	setupLog = ctrl.Log.WithName("setup")
+)
+
+func init() {
+	utilruntime.Must(clientgoscheme.AddToScheme(scheme))
+	utilruntime.Must(helmv2.AddToScheme(scheme))
+
+	// +kubebuilder:scaffold:scheme
+}
+
+func main() {
+	var metricsAddr string
+	var enableLeaderElection bool
+	var probeAddr string
+	var secureMetrics bool
+	var enableHTTP2 bool
+	var tlsOpts []func(*tls.Config)
+
+	flag.StringVar(&metricsAddr, "metrics-bind-address", "0", "The address the metrics endpoint binds to. "+
+		"Use :8443 for HTTPS or :8080 for HTTP, or leave as 0 to disable the metrics service.")
+	flag.StringVar(&probeAddr, "health-probe-bind-address", ":8081", "The address the probe endpoint binds to.")
+	flag.BoolVar(&enableLeaderElection, "leader-elect", false,
+		"Enable leader election for controller manager. "+
+			"Enabling this will ensure there is only one active controller manager.")
+	flag.BoolVar(&secureMetrics, "metrics-secure", true,
+		"If set, the metrics endpoint is served securely via HTTPS. Use --metrics-secure=false to use HTTP instead.")
+	flag.BoolVar(&enableHTTP2, "enable-http2", false,
+		"If set, HTTP/2 will be enabled for the metrics server")
+
+	opts := zap.Options{
+		Development: false,
+	}
+	opts.BindFlags(flag.CommandLine)
+	flag.Parse()
+
+	ctrl.SetLogger(zap.New(zap.UseFlagOptions(&opts)))
+
+	// if the enable-http2 flag is false (the default), http/2 should be disabled
+	// due to its vulnerabilities. More specifically, disabling http/2 will
+	// prevent from being vulnerable to the HTTP/2 Stream Cancellation and
+	// Rapid Reset CVEs. For more information see:
+	// - https://github.com/advisories/GHSA-qppj-fm5r-hxr3
+	// - https://github.com/advisories/GHSA-4374-p667-p6c8
+	disableHTTP2 := func(c *tls.Config) {
+		setupLog.Info("disabling http/2")
+		c.NextProtos = []string{"http/1.1"}
+	}
+
+	if !enableHTTP2 {
+		tlsOpts = append(tlsOpts, disableHTTP2)
+	}
+
+	// Metrics endpoint is enabled in 'config/default/kustomization.yaml'. The Metrics options configure the server.
+	// More info:
+	// - https://pkg.go.dev/sigs.k8s.io/controller-runtime@v0.19.1/pkg/metrics/server
+	// - https://book.kubebuilder.io/reference/metrics.html
+	metricsServerOptions := metricsserver.Options{
+		BindAddress:   metricsAddr,
+		SecureServing: secureMetrics,
+		TLSOpts:       tlsOpts,
+	}
+
+	mgr, err := ctrl.NewManager(ctrl.GetConfigOrDie(), ctrl.Options{
+		Scheme:                 scheme,
+		Metrics:                metricsServerOptions,
+		HealthProbeBindAddress: probeAddr,
+		LeaderElection:         enableLeaderElection,
+		LeaderElectionID:       "flux-plunger.cozystack.io",
+		// LeaderElectionReleaseOnCancel defines if the leader should step down voluntarily
+		// when the Manager ends. This requires the binary to immediately end when the
+		// Manager is stopped, otherwise, this setting is unsafe. Setting this significantly
+		// speeds up voluntary leader transitions as the new leader don't have to wait
+		// LeaseDuration time first.
+		//
+		// In the default scaffold provided, the program ends immediately after
+		// the manager stops, so would be fine to enable this option. However,
+		// if you are doing or is intended to do any operation such as perform cleanups
+		// after the manager stops then its usage might be unsafe.
+		// LeaderElectionReleaseOnCancel: true,
+	})
+	if err != nil {
+		setupLog.Error(err, "unable to create manager")
+		os.Exit(1)
+	}
+
+	if err = (&fluxplunger.FluxPlunger{
+		Client: mgr.GetClient(),
+	}).SetupWithManager(mgr); err != nil {
+		setupLog.Error(err, "unable to create controller", "controller", "FluxPlunger")
+		os.Exit(1)
+	}
+
+	// +kubebuilder:scaffold:builder
+
+	if err := mgr.AddHealthzCheck("healthz", healthz.Ping); err != nil {
+		setupLog.Error(err, "unable to set up health check")
+		os.Exit(1)
+	}
+	if err := mgr.AddReadyzCheck("readyz", healthz.Ping); err != nil {
+		setupLog.Error(err, "unable to set up ready check")
+		os.Exit(1)
+	}
+
+	setupLog.Info("starting manager")
+	if err := mgr.Start(ctrl.SetupSignalHandler()); err != nil {
+		setupLog.Error(err, "problem running manager")
+		os.Exit(1)
+	}
+}