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/appexport/export.go

@@ -14,7 +14,7 @@ import (
 	"sync"
 	"time"
 
-	"gitea.dooplex.hu/admin/felhom-controller/internal/backup"
+	"gitea.dooplex.hu/admin/felhom-controller/internal/appbackup"
 )
 
 // Step tracks one step of an export/import operation.
@@ -510,7 +510,7 @@ func (e *Exporter) dumpDatabase(stackName, dbDir string, manifest *Manifest) boo
 	defer cancel()
 
 	e.debugf("discovering databases (looking for stack %s)...", stackName)
-	dbs, err := backup.DiscoverDatabases(ctx, e.logger, e.debug)
+	dbs, err := appbackup.DiscoverDatabases(ctx, e.logger, e.debug)
 	if err != nil {
 		e.logger.Printf("[WARN] Export: DB discovery error: %v", err)
 		return false
@@ -520,7 +520,7 @@ func (e *Exporter) dumpDatabase(stackName, dbDir string, manifest *Manifest) boo
 		e.debugf("  db[%d]: stack=%s container=%s type=%s", i, dbs[i].StackName, dbs[i].ContainerName, dbs[i].DBType)
 	}
 
-	var stackDB *backup.DiscoveredDB
+	var stackDB *appbackup.DiscoveredDB
 	for i := range dbs {
 		if dbs[i].StackName == stackName {
 			stackDB = &dbs[i]
@@ -534,7 +534,7 @@ func (e *Exporter) dumpDatabase(stackName, dbDir string, manifest *Manifest) boo
 	e.debugf("matched DB: container=%s type=%s", stackDB.ContainerName, stackDB.DBType)
 
 	dumpStart := time.Now()
-	result := backup.DumpOne(ctx, *stackDB, dbDir, e.logger, e.debug)
+	result := appbackup.DumpOne(ctx, *stackDB, dbDir, e.logger, e.debug)
 	if result.Error != nil {
 		e.logger.Printf("[WARN] Export: DB dump failed for %s: %v", stackName, result.Error)
 		return false