fix: avoid double disk reserve for staged uploads (#554)

Issue: #554
User-Visible: yes

docs/CHANGELOG.md

@@ -2,6 +2,11 @@
 
 ## Unreleased
 
+- Uploading an attachment no longer reports low disk space merely because its
+  completed staging file was counted a second time. The 512 MiB safety reserve
+  itself is unchanged
+  ([#554](https://github.com/Matysh/houseplan-card/issues/554)).
+
 - Range and zone radar sources now stay live in the setup preview and use the
   same complete source list for Home Assistant read-permission checks as for
   frame calculation

docs/CHANGELOG.ru.md

@@ -8,6 +8,11 @@
 
 ## Не выпущено
 
+- Загрузка вложения больше не сообщает о нехватке места только потому, что уже
+  записанный staging-файл посчитали повторно. Сам резерв безопасности 512 МиБ
+  не изменился
+  ([#554](https://github.com/Matysh/houseplan-card/issues/554)).
+
 - Источники диапазонов и зон радара теперь обновляются в предпросмотре
   настройки, а проверка прав Home Assistant использует тот же полный список
   источников, что и расчёт данных радара

docs/TESTING.md

@@ -1652,6 +1652,13 @@ separately promised workflows:
       returns the newest 60 with a total
       [auto: unit: test_check_quota_counts_the_whole_store_not_one_request,
       backend test_uploads_are_bounded_by_a_store_quota]
+- [ ] An attachment already written to staging is not reserved twice: with the
+      real 512 MiB disk reserve intact its same-filesystem promotion succeeds,
+      while one byte below the reserve still fails. Uploads checked before any
+      bytes are written continue to reserve their full incoming size
+      [auto: backend test_issue_554_low_disk_reserve_distinguishes_staged_and_unwritten_bytes
+      + test_issue_554_upload_uses_actual_free_space_after_staging; mutation:
+      quota-reserves-staged-bytes-twice-on-disk]
 - [ ] Square canvas migration (v1.48.0): after the upgrade every existing plan
       looks exactly as before, just with margins where the canvas was extended.
       Measure a wall in the plan editor — the length in cm is unchanged. Marker