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/cmd/server/router_test.go

@@ -9,6 +9,7 @@ import (
 var expectedRoutes = []string{
 	"GET /api/v1/health",
 	"POST /api/v1/devices",
+	"POST /api/v1/auth/login",
 
 	"GET /api/v1/files",
 	"GET /api/v1/files/:id",
@@ -17,6 +18,9 @@ var expectedRoutes = []string{
 	"GET /api/v1/files/folders",
 	"POST /api/v1/files/upload",
 
+	"GET /api/v1/users/resolve",
+	"PATCH /api/v1/users/me/password",
+
 	"POST /api/v1/ocr/jobs",
 	"GET /api/v1/ocr/jobs/:id",