Have the scanner's report task ensure handovers only occur if Batchs are valid

This is incomplete at this time. The logic is fine, but needs to be moved to a
distinct location to handle singular blocks which produce multiple Batches.

processor/scanner/src/substrate/db.rs

@@ -40,6 +40,12 @@ pub(crate) enum Action<S: ScannerFeed> {
   QueueBurns(Vec<OutInstructionWithBalance>),
 }
 
+create_db!(
+  ScannerSubstrate {
+    LastAcknowledgedBatch: () -> u32,
+  }
+);
+
 db_channel!(
   ScannerSubstrate {
     Actions: () -> ActionEncodable,
@@ -48,6 +54,14 @@ db_channel!(
 
 pub(crate) struct SubstrateDb<S: ScannerFeed>(PhantomData<S>);
 impl<S: ScannerFeed> SubstrateDb<S> {
+  pub(crate) fn last_acknowledged_batch(getter: &impl Get) -> Option<u32> {
+    LastAcknowledgedBatch::get(getter)
+  }
+
+  pub(crate) fn set_last_acknowledged_batch(txn: &mut impl DbTxn, id: u32) {
+    LastAcknowledgedBatch::set(txn, &id)
+  }
+
   pub(crate) fn queue_acknowledge_batch(
     txn: &mut impl DbTxn,
     batch_id: u32,

processor/scanner/src/substrate/mod.rs

@@ -1,6 +1,6 @@
 use core::{marker::PhantomData, future::Future};
 
-use serai_db::{DbTxn, Db};
+use serai_db::{Get, DbTxn, Db};
 
 use serai_coins_primitives::{OutInstruction, OutInstructionWithBalance};
 use serai_validator_sets_primitives::Session;
@@ -14,6 +14,9 @@ use crate::{
 mod db;
 use db::*;
 
+pub(crate) fn last_acknowledged_batch<S: ScannerFeed>(getter: &impl Get) -> Option<u32> {
+  SubstrateDb::<S>::last_acknowledged_batch(getter)
+}
 pub(crate) fn queue_acknowledge_batch<S: ScannerFeed>(
   txn: &mut impl DbTxn,
   batch_id: u32,
@@ -99,6 +102,7 @@ impl<D: Db, S: ScannerFeed> ContinuallyRan for SubstrateTask<D, S> {
               "batch acknowledged on-chain was distinct"
             );
 
+            SubstrateDb::<S>::set_last_acknowledged_batch(&mut txn, batch_id);
             AcknowledgedBatches::send(
               &mut txn,
               &external_key_for_session_to_sign_batch.0,