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/storage/migrate.go

@@ -13,7 +13,7 @@ import (
 	"sync"
 	"time"
 
-	"gitea.dooplex.hu/admin/felhom-controller/internal/backup"
+	"gitea.dooplex.hu/admin/felhom-controller/internal/appbackup"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/settings"
 )
 
@@ -404,8 +404,8 @@ func (o *MigrateOrchestrator) RunEnhancedMigration(
 	// --- Post-migration steps (all non-fatal) ---
 
 	// 1. Copy DB dumps from source to destination
-	srcDBDumps := backup.AppDBDumpPath(req.CurrentHDDPath, req.StackName)
-	dstDBDumps := backup.AppDBDumpPath(req.TargetPath, req.StackName)
+	srcDBDumps := appbackup.AppDBDumpPath(req.CurrentHDDPath, req.StackName)
+	dstDBDumps := appbackup.AppDBDumpPath(req.TargetPath, req.StackName)
 	if _, err := os.Stat(srcDBDumps); err == nil {
 		if err := os.MkdirAll(filepath.Dir(dstDBDumps), 0755); err != nil {
 			o.Logger.Printf("[WARN] [storage] Migration %s: failed to create DB dump dir: %v", req.StackName, err)