refactor: extract app-data-backup into internal/appbackup (no behaviour change)

Extract the stateless, keep-side app-data backup primitives out of
internal/backup/ into a new self-contained internal/appbackup/ package:
- dbdump.go: DB dump discovery/execution (DiscoverDatabases, DumpOne, ...)
- appdata.go: StackDataProvider + app-data/volume discovery, HumanizeBytes
- paths.go: keep-side path helpers (AppDBDumpPath, AppVolumeDumpPath, AppDataDir)

backup/ keeps every name available via type/const aliases + one-line function
forwarders (appbackup_bridge.go), so the still-present delete-side code
(restic, cross-drive, drive-mount) and the both-side consumers (web/api/report)
compile unchanged. The keep-only consumers appexport and storage are rewired to
import appbackup directly and no longer import backup.

This is the Part-2 prerequisite for the Proxmox port: appbackup has zero
references to restic/cross-drive/drive-mount and does not import backup, so the
delete-side can later be removed without breaking app-data backup or appexport.

Behaviour-preserving: pure move + import/qualifier rewrites, no logic edits.
The four Manager methods (RunDBDumps/DumpAppVolumes/DumpAppVolumesSafe share the
delete-side mutex/status state; RestoreAppFromTier2 reads the cross-drive mirror)
intentionally stay on Manager and delegate to appbackup — for the re-platform step.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

controller/internal/appbackup/paths.go

@@ -0,0 +1,31 @@
+// Package appbackup holds the self-contained app-data backup primitives:
+// database dump and Docker-volume archive discovery/execution, plus the
+// keep-side storage path helpers. It depends only on stable abstractions
+// (the StackDataProvider interface) and has no dependency on the restic,
+// cross-drive, or drive-mount code in the backup package.
+package appbackup
+
+import "path/filepath"
+
+// FelhomDataDir is the namespace directory on storage drives for all felhom-managed data.
+const FelhomDataDir = "felhom-data"
+
+// PrimaryBackupPath returns the root primary backup directory for a drive.
+func PrimaryBackupPath(drivePath string) string {
+	return filepath.Join(drivePath, FelhomDataDir, "backups", "primary")
+}
+
+// AppDBDumpPath returns the DB dump directory for an app on its home drive.
+func AppDBDumpPath(drivePath, stackName string) string {
+	return filepath.Join(drivePath, FelhomDataDir, "backups", "primary", stackName, "db-dumps")
+}
+
+// AppVolumeDumpPath returns the directory for Docker volume dump tars on an app's home drive.
+func AppVolumeDumpPath(drivePath, stackName string) string {
+	return filepath.Join(drivePath, FelhomDataDir, "backups", "primary", stackName, "volume-dumps")
+}
+
+// AppDataDir returns the app data directory path on a drive.
+func AppDataDir(drivePath, stackName string) string {
+	return filepath.Join(drivePath, FelhomDataDir, "appdata", stackName)
+}