fix: P0+P1 critical bug fixes across controller (24 files)

Concurrency fixes:
- Deep-copy stacks in GetStack/GetStacks to prevent shared state mutation (C04)
- Add per-state mutex to watchdog pathProbeState (C05)
- Guard MetricsCollector.Start() with sync.Once against double-start (C06)
- Hold diskJobMu across entire raw mount operation (C07)
- Add mutex to SetEncryptionKey (C08), MigrateEncryption write lock (H03)
- Use sync.Once for sync.Stop() channel close (H08)
- Set syncing=true before releasing lock in TriggerSync (H09)
- Deep-copy lastDBDump/lastBackup in GetFullStatus (H11)
- Add WaitGroup for stderr goroutine in MigrateDrive (H19)
- Add mutex to SetBackupRunningCheck (M18)

Security fixes:
- Validate Bearer token against Hub API key in CSRF middleware (H16)
- Validate backup paths start with expected prefix in RemoveStack (M12)
- Guard uuid[:8] slice with length check (H20)
- Parse fstab fields exactly for mount target matching (H21)

Bug fixes:
- Use decrypted env vars for compose deploy (C01)
- Log decrypt failures in DecryptMap instead of swallowing (C02)
- Move Deployed=false inside lock in runComposeDeploy (C03)
- Fix activeDrives() to skip disconnected drives (H02)
- Fix Snapshot() stderr extraction from exec.ExitError (H01)
- Check unlockCmd.Run() error in restic (H01)
- Buffer template rendering via bytes.Buffer (H07)
- Thread context.Context through cloudflare client (H10)
- Fix leaf-name collision detection in cross-drive backup (H15)
- Add nil check for crossDriveRunner (H17)
- Use strings.TrimSpace instead of slice on command output (H18)
- Make SaveAppConfig atomic with write-to-tmp+rename (H04)
- Pass encKey on deploy failure SaveAppConfig (H05)
- Fix IPv6 address format in TCP health probe

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/metrics/collector.go

@@ -7,6 +7,7 @@ import (
 	"os/exec"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"gitea.dooplex.hu/admin/felhom-controller/internal/system"
@@ -19,6 +20,7 @@ type MetricsCollector struct {
 	hddPath      string
 	logger       *log.Logger
 	cancel       context.CancelFunc
+	startOnce    sync.Once
 }
 
 // NewMetricsCollector creates a new collector.
@@ -32,9 +34,12 @@ func NewMetricsCollector(store *MetricsStore, cpuCollector *system.CPUCollector,
 }
 
 // Start begins the background collection loop (every 60 seconds).
+// Safe to call multiple times — only the first call starts the loop.
 func (c *MetricsCollector) Start(ctx context.Context) {
-	ctx, c.cancel = context.WithCancel(ctx)
-	go c.loop(ctx)
+	c.startOnce.Do(func() {
+		ctx, c.cancel = context.WithCancel(ctx)
+		go c.loop(ctx)
+	})
 }
 
 // Stop cancels the collection loop.