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>

scripts/docker-setup.sh

@@ -1280,7 +1280,7 @@ install_filebrowser() {
 # Deployed by docker-setup.sh — do NOT remove
 #
 # Mount permissions:
-#   /srv/storage/       → HDD storage/ (READ-ONLY — app data)
+#   /srv/appdata/       → HDD appdata/ (READ-ONLY — app data)
 #   /srv/media/         → HDD media/   (read-write — user media)
 #   /srv/Dokumentumok/  → HDD Dokumentumok/ (read-write — user documents)
 
@@ -1293,7 +1293,7 @@ services:
       - TZ=Europe/Budapest
     volumes:
       - filebrowser_data:/home/filebrowser/data
-      - \${HDD_PATH}/storage:/srv/storage:ro
+      - \${HDD_PATH}/appdata:/srv/appdata:ro
       - \${HDD_PATH}/media:/srv/media
       - \${HDD_PATH}/Dokumentumok:/srv/Dokumentumok
     networks: