feat(admin): online dictionary update — upload archive, preview word diff, install & activate

Replace the dictionary hot-reload with an online update flow in the GM console
(/_gm/dictionary): the operator uploads scrabble-dawg-vX.Y.Z.tar.gz, previews the
per-variant words added/removed against the active dictionary, and confirms to
install + activate it. Versions are immutable; in-progress games keep their pinned
version while new games use the new one.

- engine: DiffWords (enumerate both DAWGs, decode only the differences), OpenFinder,
  Registry.Finder, DictFiles; OpenWithVersions skips the .staging area.
- dictadmin: hardened release-archive validation + extraction (path-traversal,
  symlink, oversize, entry-count rejection) and staging -> install (atomic rename).
- game: active dictionary version persisted in the dictionary_state singleton
  (single source of truth, restored on boot), concurrency-safe accessor.
- storage: BACKEND_DICT_DIR is a named volume seeded from the image (nonroot-owned),
  so uploaded versions persist across redeploys; the build's DICT_VERSION labels the
  seed and equals the resident tag (BACKEND_DICT_VERSION).
- docs: ARCHITECTURE §5, FUNCTIONAL (+ru), backend README, TESTING, PRERELEASE.

Tests: engine + dictadmin unit; integration upload->preview->install->activate->
restart->pin->immutability->CSRF.

backend/internal/engine/registry.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
+	"strings"
 	"sync"
 
 	dawg "github.com/iliadenisov/dafsa"
@@ -83,7 +84,9 @@ func OpenWithVersions(dir, bootVersion string) (*Registry, error) {
 		return nil, fmt.Errorf("engine: scan dictionary dir %s: %w", dir, err)
 	}
 	for _, e := range entries {
-		if !e.IsDir() || e.Name() == bootVersion {
+		// Skip non-directories, the boot version (already loaded as the flat dir)
+		// and dot-prefixed directories (the upload staging area, dir/.staging/).
+		if !e.IsDir() || e.Name() == bootVersion || strings.HasPrefix(e.Name(), ".") {
 			continue
 		}
 		if _, err := r.LoadAvailable(filepath.Join(dir, e.Name()), e.Name()); err != nil {