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/paths.go

@@ -0,0 +1,38 @@
+package backup
+
+import "path/filepath"
+
+// PrimaryBackupPath returns the root primary backup directory for a drive.
+func PrimaryBackupPath(drivePath string) string {
+	return filepath.Join(drivePath, "backups", "primary")
+}
+
+// PrimaryResticRepoPath returns the restic repo path on a drive's primary backup.
+func PrimaryResticRepoPath(drivePath string) string {
+	return filepath.Join(drivePath, "backups", "primary", "restic")
+}
+
+// AppDBDumpPath returns the DB dump directory for an app on its home drive.
+func AppDBDumpPath(drivePath, stackName string) string {
+	return filepath.Join(drivePath, "backups", "primary", stackName, "db-dumps")
+}
+
+// SecondaryBackupPath returns the root secondary backup directory for a drive.
+func SecondaryBackupPath(drivePath string) string {
+	return filepath.Join(drivePath, "backups", "secondary")
+}
+
+// AppSecondaryRsyncPath returns the rsync destination for an app's secondary backup.
+func AppSecondaryRsyncPath(drivePath, stackName string) string {
+	return filepath.Join(drivePath, "backups", "secondary", stackName, "rsync")
+}
+
+// SecondaryResticRepoPath returns the restic repo path on a drive's secondary backup.
+func SecondaryResticRepoPath(drivePath string) string {
+	return filepath.Join(drivePath, "backups", "secondary", "restic")
+}
+
+// AppDataDir returns the app data directory path on a drive.
+func AppDataDir(drivePath, stackName string) string {
+	return filepath.Join(drivePath, "appdata", stackName)
+}