fix: add CHECK constraint on file_size > 0

Prevents negative or zero file_size values at the database level.
Without this, corrupted data from import or direct DB access could
cause incorrect storage accounting (getUsedStorage sums file_size,
and expiredFiles casts to Word32 which wraps negative values).

src/Simplex/FileTransfer/Server/Store/Postgres/Migrations.hs

@@ -14,7 +14,8 @@ import Text.RawString.QQ (r)
 
 xftpSchemaMigrations :: [(String, Text, Maybe Text)]
 xftpSchemaMigrations =
-  [ ("20260325_initial", m20260325_initial, Nothing)
+  [ ("20260325_initial", m20260325_initial, Nothing),
+    ("20260402_file_size_check", m20260402_file_size_check, Just down_m20260402_file_size_check)
   ]
 
 -- | The list of migrations in ascending order by date
@@ -45,3 +46,15 @@ CREATE TABLE recipients (
 CREATE INDEX idx_recipients_sender_id ON recipients (sender_id);
 CREATE INDEX idx_files_created_at ON files (created_at);
 |]
+
+m20260402_file_size_check :: Text
+m20260402_file_size_check =
+  [r|
+ALTER TABLE files ADD CONSTRAINT check_file_size_positive CHECK (file_size > 0);
+|]
+
+down_m20260402_file_size_check :: Text
+down_m20260402_file_size_check =
+  [r|
+ALTER TABLE files DROP CONSTRAINT check_file_size_positive;
+|]