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/backup/restic.go

@@ -134,12 +134,17 @@ func (r *ResticManager) Snapshot(repoPath string, paths []string, tags []string)
 	cmd := r.command(ctx, repoPath, args...)
 	out, err := cmd.Output()
 	if err != nil {
-		// Check for stale lock
+		// Check for stale lock — restic writes lock errors to stderr, not stdout
 		errStr := string(out)
+		if exitErr, ok := err.(*exec.ExitError); ok {
+			errStr += string(exitErr.Stderr)
+		}
 		if strings.Contains(errStr, "lock") || strings.Contains(errStr, "locked") {
 			r.logger.Printf("[WARN] Restic repo locked — attempting unlock")
 			unlockCmd := r.command(ctx, repoPath, "unlock")
-			unlockCmd.Run()
+			if unlockErr := unlockCmd.Run(); unlockErr != nil {
+				r.logger.Printf("[WARN] Restic unlock failed: %v", unlockErr)
+			}
 			// Retry once
 			cmd = r.command(ctx, repoPath, args...)
 			out, err = cmd.Output()