F-S2 + F-S3: compose-derived appdata dir resolution (v0.131.0)

The controller assumed an app's HDD appdata dir is always appdata/<stackName>.
paperless-ngx writes appdata/paperless (stack paperless-ngx), so every consumer
keying by stack name silently missed it via a stat-and-skip. One canonical
resolver appbackup.AppDataDirNames derives the real dir name(s) from the app's
compose ${HDD_PATH} binds; all consumers use it.

- F-S2 (tier-2): RunTier2 mirrors the resolved appdata/<name> (paperless docs
  got NO tier-2 copy before). Tier2Info size + RestoreTier2Files live dir use it.
  WARN when a declared appdata dir is absent. New tier2Mirror seam.
- F-S3 (migrate, NEW): all six per-app appdata legs (collision/size/copy/verify/
  cleanup/skip-set) now loop resolved names. scope="app" migration of paperless
  previously copied nothing and left an empty media dir (scope="all" was saved by
  the merge walk). WARN on missing declared dir in the copy leg.
- Multi-dir (N>1) refusal: tier-2 backup/info/restore refuse loudly (Hungarian);
  migrate supports N. No catalog app hits it today; lifted by Task 3.
- Display: storage page sums resolved dirs.
- Truth repair: the v0.130.0 "tier-2 copies the namespace wholesale" claim is
  false; corrected in CHANGELOG + main.go export-adapter comment.

+9 tests; red-proofs RP-1..RP-5 all confirmed. Controller-only, no agent/hub
coupling. Task 1 of the backup-classification-redesign arc.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A45Qop8YY8tS94bz63LFne

controller/internal/backup/backup.go

@@ -65,6 +65,11 @@ type Manager struct {
 	// the orchestration never shells out. Nil → the real rsyncRestoreMissing (additive-only).
 	restoreFilesCopier func(src, dst string) (filesRestored int, err error)
 
+	// tier2Mirror (F-S2) — the Tier-2 backup mirror seam (both rsync legs in RunTier2), overridable
+	// so the resolve→mirror→record flow is unit-testable without rsync. Nil → the real rsyncMirror
+	// (`-a --delete`, contents-of-src semantics).
+	tier2Mirror func(src, dst string) error
+
 	// migrationRunning, if set, reports whether a data migration is in progress. The scheduled
 	// backup paths skip when it returns true (Change 3 — backup ↔ migration mutual exclusion), so a
 	// nightly dump/Tier-2 can't race a migration copy/cleanup on the same drive.