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/backup/paths.go

@@ -2,29 +2,15 @@ package backup
 
 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")
-}
+// Keep-side path helpers (FelhomDataDir, PrimaryBackupPath, AppDBDumpPath,
+// AppVolumeDumpPath, AppDataDir) now live in internal/appbackup and are
+// re-exposed here via aliases/forwarders in appbackup_bridge.go.
 
 // PrimaryResticRepoPath returns the restic repo path on a drive's primary backup.
 func PrimaryResticRepoPath(drivePath string) string {
 	return filepath.Join(drivePath, FelhomDataDir, "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, 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")
-}
-
 // SecondaryBackupPath returns the root secondary backup directory for a drive.
 func SecondaryBackupPath(drivePath string) string {
 	return filepath.Join(drivePath, FelhomDataDir, "backups", "secondary")
@@ -45,11 +31,6 @@ func SecondaryInfraPath(drivePath string) string {
 	return filepath.Join(drivePath, FelhomDataDir, "backups", "secondary", "_infra")
 }
 
-// AppDataDir returns the app data directory path on a drive.
-func AppDataDir(drivePath, stackName string) string {
-	return filepath.Join(drivePath, FelhomDataDir, "appdata", stackName)
-}
-
 // InfraBackupDir returns the hidden infra backup directory on a drive.
 func InfraBackupDir(mountPath string) string {
 	return filepath.Join(mountPath, ".felhom-infra-backup")