feat(m10): file sharing with upload, lightbox, permission, tests, e2e

- FileRepository (CRUD, soft-delete, pagination, project isolation) +
  FileStorageService (local FS save data/uploads/<projectId>/<uuid>.<ext>,
  MIME allowlist, filename sanitize, unique uuid name, member permission,
  file_shared notification to other members, file_uploaded activity log,
  SSE file.uploaded, uploader/admin delete + FS unlink)
- APIs: files list/upload(multipart), download (membership), delete
- Screens: files page + Uploader(multipart) + FileList(image Lightbox,
  PDF link, delete), ProjectNav ファイル link
- Unit tests (FileRepository, FileStorageService) + e2e file-sharing

components/layout/ProjectNav.tsx

@@ -4,6 +4,7 @@ const NAV_ITEMS = [
   { href: '/notes', label: 'メモ' },
   { href: '/chat', label: 'チャット' },
   { href: '/todos', label: 'ToDo' },
+  { href: '/files', label: 'ファイル' },
   { href: '/members', label: 'メンバー' },
   { href: '/activity', label: 'アクティビティ' },
   { href: '/settings', label: '設定' },
@@ -24,6 +25,7 @@ export function ProjectNav({
     | 'notes'
     | 'chat'
     | 'todos'
+    | 'files'
     | 'members'
     | 'activity'
     | 'settings';
@@ -34,6 +36,7 @@ export function ProjectNav({
     notes: active === 'notes',
     chat: active === 'chat',
     todos: active === 'todos',
+    files: active === 'files',
     members: active === 'members',
     activity: active === 'activity',
     settings: active === 'settings',