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/appbackup/paths.go

@@ -5,7 +5,11 @@
 // cross-drive, or drive-mount code in the backup package.
 package appbackup
 
-import "path/filepath"
+import (
+	"path/filepath"
+	"sort"
+	"strings"
+)
 
 // FelhomDataDir is the namespace directory on storage drives for all felhom-managed data.
 const FelhomDataDir = "felhom-data"
@@ -64,7 +68,61 @@ func AppVolumeDumpPath(nsRoot, stackName string) string {
 	return filepath.Join(RecoveryUnitPath(nsRoot, stackName), "volume-dumps")
 }
 
-// AppDataDir returns the app data directory under a felhom-data namespace root.
+// AppDataDir returns the app data directory under a felhom-data namespace root. The final segment
+// is the app's real appdata dir NAME — usually the stack name, but NOT always: paperless-ngx writes
+// appdata/paperless (F-S2/F-S3). Callers that key by stack name silently miss such apps; use
+// AppDataDirNames to resolve the real name(s) from the app's compose binds and pass them here.
 func AppDataDir(nsRoot, stackName string) string {
 	return filepath.Join(nsRoot, "appdata", stackName)
 }
+
+// AppDataDirNames returns the app's real directory name(s) under <hddPath>/appdata, derived from its
+// compose HDD bind mounts (F-S2/F-S3: the dir name is NOT always the stack name — paperless-ngx
+// writes appdata/paperless). hddMounts are resolved host paths in the ParseComposeHDDMounts shape
+// (each is <hddPath> itself or a subpath, filepath.Clean'd). The first path element under
+// <hddPath>/appdata/ is taken as the dir name; results are deduped and sorted. Falls back to
+// []string{stackName} when no appdata-prefixed mount is derivable (no HDD appdata binds, unreadable
+// compose, nil provider) — the exact legacy behavior.
+//
+// Today every catalog app resolves to exactly ONE name (immich→immich, nextcloud→nextcloud,
+// romm→romm, paperless-ngx→paperless). The N>1 return is defensive: tier-2 refuses it loudly,
+// migrate handles it naturally.
+func AppDataDirNames(hddPath, stackName string, hddMounts []string) []string {
+	prefix := filepath.Clean(hddPath) + string(filepath.Separator) + "appdata" + string(filepath.Separator)
+	seen := make(map[string]bool)
+	var names []string
+	for _, mnt := range hddMounts {
+		cm := filepath.Clean(mnt)
+		if !strings.HasPrefix(cm, prefix) {
+			continue // not under appdata/ (a whole-root bind, a different subtree, a foreign drive)
+		}
+		rem := strings.TrimPrefix(cm, prefix)
+		first := strings.Split(rem, string(filepath.Separator))[0]
+		if first == "" {
+			continue
+		}
+		if !seen[first] {
+			seen[first] = true
+			names = append(names, first)
+		}
+	}
+	if len(names) == 0 {
+		return []string{stackName}
+	}
+	sort.Strings(names)
+	return names
+}
+
+// AppDataBindsPresent reports whether any of the app's resolved HDD mounts sits under
+// <hddPath>/appdata/ — i.e. the compose actually DECLARES an appdata bind. Callers use it to
+// distinguish "no appdata to back up" (silent skip is correct) from "declared appdata dir missing
+// on disk" (the silence that hid F-S2 — worth a WARN). Same prefix rule as AppDataDirNames.
+func AppDataBindsPresent(hddPath string, hddMounts []string) bool {
+	prefix := filepath.Clean(hddPath) + string(filepath.Separator) + "appdata" + string(filepath.Separator)
+	for _, mnt := range hddMounts {
+		if strings.HasPrefix(filepath.Clean(mnt), prefix) {
+			return true
+		}
+	}
+	return false
+}