fix(sends): emit hideEmail as non-null boolean in sync response

The /api/sync response serialized a Send hide_email field directly from
Option<bool>, so a NULL value in the sends table (the column is
Nullable<Bool> with no default) produced "hideEmail": null.

The Bitwarden Android client deserializes SyncResponseJson.Send.hideEmail
as a non-null Kotlin Boolean and aborts the entire sync with a
JsonDecodingException when it encounters null. Web, desktop and CLI
clients coerce null to false, so only accounts with at least one Send
are affected and only on Android.

Default None to false at the serialization boundary, matching the
official Bitwarden server where hideEmail is non-nullable. This needs no
database migration and fixes both legacy NULL rows and any future NULLs.
The hide_email field stays Option<bool> internally.

src/db/models/send.rs

@@ -161,7 +161,7 @@ impl Send {
             "password": self.password_hash.as_deref().map(|h| BASE64URL_NOPAD.encode(h)),
             "authType": if self.password_hash.is_some() { SendAuthType::Password as i32 } else { SendAuthType::None as i32 },
             "disabled": self.disabled,
-            "hideEmail": self.hide_email,
+            "hideEmail": self.hide_email.unwrap_or(false),
 
             "revisionDate": format_date(&self.revision_date),
             "expirationDate": self.expiration_date.as_ref().map(format_date),