v0.14.0: Per-drive backup architecture + storage path overhaul

Major refactor of backup and storage paths:

- Per-drive restic repos at <drive>/backups/primary/restic/
- Per-app DB dumps at <drive>/backups/primary/<app>/db-dumps/
- Remove global BackupDir, DBDumpDir, ResticRepo config fields
- Add SystemDataPath config (fallback for apps without HDD)
- New backup/paths.go with pure path computation helpers
- Add GetStackHDDPath to StackDataProvider interface
- Restic methods now accept repoPath as parameter
- Cross-drive backup uses new secondary path structure
- Rename storage/ to appdata/ in scripts and compose templates
- Update protected HDD paths (storage → appdata + backups)
- Simplify backup UI (remove global path displays)

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

controller/internal/backup/appdata.go

@@ -16,6 +16,7 @@ type StackDataProvider interface {
 	GetStackComposePath(name string) (composePath string, ok bool)
 	ListDeployedStacks() []StackSummary
 	GetStackHDDMounts(name string) []string
+	GetStackHDDPath(name string) string // raw HDD_PATH from app.yaml (empty if no HDD)
 	StopStack(name string) error
 	StartStack(name string) error
 }