feat(api): identité user-first — login/password, ownership user, fin du token device

- 000006 : users.username/username_normalized/password_hash/is_admin, index
  unique partiel sur username_normalized, DROP idx_users_email (unicité V1 =
  username, jamais email) ; 000007 destructif : resources.owner_id→user_id
  (FK→users, reset base dev), l'unicité racine devient par user
- pkg/passwd : argon2id (t=1,m=64MiB,p=4,k=32), comparaison constant-time,
  VerifyTimedEqual (délai égalisé dummy-hash) ; pkg/auth : subject=user_id +
  claim device_id, TTL 7 j sans refresh
- repository : Users (count/get/resolve-exact/create/update-password/
  mark-deleted), Devices.MarkUser (user_id INFORMATIF uniquement)
- handlers : POST /devices sans token, POST /auth/login (401 indistinguable
  user inconnu/mauvais mdp, device requis), PATCH /users/me/password
  (current_password, tokens non révoqués — limite V1), GET /users/resolve
  (exact lowercase, jamais email/is_admin) ; middleware RequireAuth
  (user authorisant, device porté) ; tout le scoping ressources passe user
- bootstrap admin : users vide + ADMIN_* absents → refus de démarrer ;
  .env.example ; docs/api-v1.md §2/§3/§7/§8
- tests : migrations 000006/000007 (up/down), pkg/auth Identity, handlers
  login/password/resolve, helpers refactorés registerAndLogin, repo tests
  scopés user — suite backend verte

backend/service/resources.go

@@ -36,7 +36,7 @@ type FolderDTO struct {
 }
 
 // Resources holds the business logic for files/folders list-get-delete-upload,
-// always scoped by the requesting device.
+// always scoped by the requesting USER (resources.user_id after 000007).
 type Resources struct {
 	Repo        *repository.Resources
 	Repository  *repository.Repository
@@ -101,7 +101,7 @@ func (s *Resources) SearchFiles(ownerID, q string, page, pageSize int) ([]FileDT
 	return files, total, nil
 }
 
-// Upload persists the multipart-sourced file under UploadDir/<device> and
+// Upload persists the multipart-sourced file under UploadDir/<user> and
 // records its metadata, returning the FileDTO. The physical file is removed
 // if metadata persistence fails (e.g. name conflict).
 func (s *Resources) Upload(ownerID string, file *multipart.FileHeader, folderID string) (FileDTO, error) {