feat(privacy): reconnect database maintenance and the manual retention run

Two sections that had backends and no buttons.

Database: path, size, WAL size, message count, and a clear button. The
numbers refresh at most every five seconds and not at all while a long
operation owns the store -- MessageCount takes the read lock, and asking
for it during a VACUUM means waiting for the whole file to be rewritten,
on the draw thread. The old version called ClearMessages straight from
the draw thread, VACUUM included; it runs on a worker now.

One line beyond the old layout sits above the clear button: how many
messages are stored, and that exporting keeps a copy. Whoever is about
to throw the history away should be told there is a way not to.

The legacy Chat 2 files only get a block when they are actually on disk,
and the advanced tools only appear when the section is expanded with
Shift held. The message injector is not back: it was deleted with the
tab and writing 10,000 fake messages into a user's real database is not
something to rebuild on the way past.

Retention: an "apply now" button, the running hint, and the last-run
line, which v1.11.0 shipped as an English literal while both strings sat
translated in all 25 languages. Plus reset-to-spec next to the existing
clear-overrides, since the two answer different questions and both were
already translated.

Retention_Apply_Tooltip stays unused and gets a replacement. It ends
with "Save your changes first", and the window it was written for had a
Save button.

Also here, found while wiring the manual trigger:

DbOperationGate.End now takes the operation it releases. It used to
reset blindly, on the reasoning that a worker must be able to release
from a finally without knowing whether it acquired. That is backwards: a
worker whose TryBegin was refused also runs its finally, and a blind
reset there hands away the lock of whichever operation actually holds
it. Worse than no gate, because the refused worker walks off believing
it did nothing while a VACUUM starts under somebody's open reader.

HellionChat/Resources/HellionStrings.pt-PT.resx

@@ -1177,4 +1177,10 @@
   <data name="Cleanup_Unavailable_NothingListed" xml:space="preserve">
     <value>Não está selecionado nenhum canal, por isso uma limpeza apagaria todo o histórico. Escolha os canais que quer manter, ou use o botão de apagar se quiser mesmo remover tudo.</value>
   </data>
+  <data name="Settings_Database_ClearHint" xml:space="preserve">
+    <value>Estão guardadas {0:N0} mensagens. Se quiser manter uma cópia, exporte-as antes de apagar.</value>
+  </data>
+  <data name="Retention_RunNow_Tooltip" xml:space="preserve">
+    <value>Ctrl+Shift: executa a limpeza de retenção agora em vez de esperar a varredura diária. Apaga mensagens mais antigas do que os limites acima.</value>
+  </data>
 </root>