v0.14.1: Auto Tier 2 for small apps + infra config in cross-drive backup

- Auto-enable daily rsync Tier 2 for apps without HDD mounts when ≥2
  storage paths exist (AutoEnableSmallApps)
- Sync infrastructure config (stacks dir + controller.yaml) to all
  secondary destinations via _infra/ directory (syncInfraConfig)
- Include infra paths in cross-drive restic snapshots
- Add SecondaryInfraPath() helper to paths.go

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

controller/internal/backup/paths.go

@@ -32,6 +32,11 @@ func SecondaryResticRepoPath(drivePath string) string {
 	return filepath.Join(drivePath, "backups", "secondary", "restic")
 }
 
+// SecondaryInfraPath returns the infrastructure config mirror directory on a drive's secondary backup.
+func SecondaryInfraPath(drivePath string) string {
+	return filepath.Join(drivePath, "backups", "secondary", "_infra")
+}
+
 // AppDataDir returns the app data directory path on a drive.
 func AppDataDir(drivePath, stackName string) string {
 	return filepath.Join(drivePath, "appdata", stackName)