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/handlers/sync.go

@@ -18,7 +18,7 @@ func SyncOpsPush(c *gin.Context) {
 		api.Error(c, http.StatusServiceUnavailable, "SERVICE_UNAVAILABLE", "backend not initialized")
 		return
 	}
-	deviceID := c.GetString(DeviceIDKey)
+	userID := c.GetString(UserIDKey)
 
 	var req syncOpsRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
@@ -26,7 +26,7 @@ func SyncOpsPush(c *gin.Context) {
 		return
 	}
 
-	result, err := Store.ApplyBatch(deviceID, req.Operations)
+	result, err := Store.ApplyBatch(userID, c.GetString(DeviceIDKey), req.Operations)
 	if err != nil {
 		writeError(c, err)
 		return
@@ -39,13 +39,13 @@ func SyncPermissionsGet(c *gin.Context) {
 		api.Error(c, http.StatusServiceUnavailable, "SERVICE_UNAVAILABLE", "backend not initialized")
 		return
 	}
-	deviceID := c.GetString(DeviceIDKey)
+	userID := c.GetString(UserIDKey)
 	after := c.Query("after")
 	var afterMs int64
 	if after != "" {
 		afterMs = int64(intParam(after, 0))
 	}
-	perms, err := Store.Snapshot(deviceID, afterMs)
+	perms, err := Store.Snapshot(userID, afterMs)
 	if err != nil {
 		writeError(c, err)
 		return