feat: bootstrap MediaStationGo (Go + React rewrite of MediaStation)

Adopt the cropflre/nowen-video tech stack and rebuild the project from
scratch:

  - Backend: Go 1.25 + Gin + GORM + SQLite (WAL) + JWT + WebSocket hub.
    Layered packages config / database / model / repository / service /
    middleware / handler. Default admin (admin/admin123) seeded on first
    run; /api routes for auth, libraries, media, stream and admin
    panels. WebSocket scan-progress events at /api/ws.
  - Frontend: React 18 + Vite 5 + Tailwind 3.4 + Zustand + axios +
    react-router 6 + lucide-react + framer-motion + hls.js. Pages for
    Login / Home / Library / Search / MediaDetail / Player / Admin
    (Library, Users, Settings tabs).
  - Distribution: multi-arch Dockerfile (frontend -> backend -> Alpine
    runtime), docker-compose.yml, GitHub Actions for CI and GHCR
    publish, Makefile, env-prefixed config (MEDIASTATION_*).
  - Docs: README, CONTRIBUTING, .env.example, config.example.yaml.

Backend builds, vets and tests pass. Frontend builds via tsc -b && vite
build (250 kB JS / 16 kB CSS, gzipped 84 / 4 kB).

internal/service/walk.go

@@ -0,0 +1,35 @@
+// Package service — internal filesystem walker shared by scanner / watcher.
+package service
+
+import (
+	"io/fs"
+	"path/filepath"
+)
+
+// walkInfo is a tiny abstraction over os.FileInfo so that callers do not
+// need to depend on os/io packages directly.
+type walkInfo struct {
+	isDir bool
+	size  int64
+}
+
+// walk traverses root depth-first calling fn for every entry. Hidden
+// directories (starting with ".") are skipped.
+func walk(root string, fn func(string, walkInfo) error) error {
+	return filepath.WalkDir(root, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return nil // best effort — keep walking
+		}
+		name := d.Name()
+		if d.IsDir() && name != "." && len(name) > 1 && name[0] == '.' {
+			return filepath.SkipDir
+		}
+		info := walkInfo{isDir: d.IsDir()}
+		if !d.IsDir() {
+			if fi, err := d.Info(); err == nil {
+				info.size = fi.Size()
+			}
+		}
+		return fn(path, info)
+	})
+}