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/cmd/controller/main.go

@@ -132,13 +132,12 @@ func main() {
 	}
 
 	// --- Initialize cross-drive backup runner ---
-	crossDriveRunner := backup.NewCrossDriveRunner(sett, stackProv, logger)
+	crossDriveRunner := backup.NewCrossDriveRunner(sett, stackProv, cfg.Paths.SystemDataPath, logger)
 
 	// Wire cross-drive → backup manager for pre-backup DB dumps
 	if backupMgr != nil {
 		crossDriveRunner.SetDBDumper(backupMgr)
 	}
-	crossDriveRunner.SetDBDumpDir(cfg.Paths.DBDumpDir)
 
 	// --- Initialize alert manager ---
 	alertMgr := web.NewAlertManager(logger)
@@ -449,6 +448,19 @@ func (a *stackAdapter) GetStackHDDMounts(name string) []string {
 	return allMounts
 }
 
+func (a *stackAdapter) GetStackHDDPath(name string) string {
+	s, ok := a.mgr.GetStack(name)
+	if !ok {
+		return ""
+	}
+	stackDir := filepath.Dir(s.ComposePath)
+	appCfg := stacks.LoadAppConfig(stackDir)
+	if appCfg != nil && appCfg.Env["HDD_PATH"] != "" {
+		return filepath.Clean(appCfg.Env["HDD_PATH"])
+	}
+	return ""
+}
+
 // discoverHDDPaths scans deployed apps' app.yaml for HDD_PATH env values.
 func discoverHDDPaths(stacksDir string, logger *log.Logger) []string {
 	entries, err := os.ReadDir(stacksDir)