wallet: fix migration speed for postgres.

Testing a large db shows Postgres slowing down exponentially as
it inserts the channel_events.  Rather than updating the index in the
db every time, do it at the end, for spectacular speedup:

```
lightningd-1 2025-10-08T05:39:44.333Z INFO    lightningd: Creating database
lightningd-1 2025-10-08T05:39:47.581Z DEBUG   lightningd: Transferring 6166 chain_events
lightningd-1 2025-10-08T05:39:48.455Z DEBUG   lightningd: Transferring 1660043 channel_events
lightningd-1 2025-10-08T05:39:54.390Z INFO    lightningd: Inserted 103100/1660043 channel_events
lightningd-1 2025-10-08T05:40:04.390Z INFO    lightningd: Inserted 283280/1660043 channel_events
lightningd-1 2025-10-08T05:40:14.390Z INFO    lightningd: Inserted 464065/1660043 channel_events
lightningd-1 2025-10-08T05:40:24.390Z INFO    lightningd: Inserted 629559/1660043 channel_events
lightningd-1 2025-10-08T05:40:34.390Z INFO    lightningd: Inserted 800659/1660043 channel_events
lightningd-1 2025-10-08T05:40:44.390Z INFO    lightningd: Inserted 975433/1660043 channel_events
lightningd-1 2025-10-08T05:40:54.390Z INFO    lightningd: Inserted 1134719/1660043 channel_events
lightningd-1 2025-10-08T05:41:04.390Z INFO    lightningd: Inserted 1290549/1660043 channel_events
lightningd-1 2025-10-08T05:41:14.390Z INFO    lightningd: Inserted 1443304/1660043 channel_events
lightningd-1 2025-10-08T05:41:24.390Z INFO    lightningd: Inserted 1590013/1660043 channel_events
lightningd-1 2025-10-08T05:41:29.148Z INFO    lightningd: bookkeeper migration complete: migrated 6166 chainmoves, 1660043 channelmoves, 132481 descriptions
```

Now we complete the entire migration in 1 minute 45 seconds.

Thanks to @Michael1101 for reporting this.

Signed-off-by: Rusty Russell <[email protected]>
Changelog-Fixed: db: migration from v25.09 on a reasonable size account database could take almost infinite time.

Author: rustyrussell

wallet/account_migration.c

@@ -366,6 +366,7 @@ void migrate_from_account_db(struct lightningd *ld, struct db *db)
 	struct db_stmt *stmt;
 	int version;
 	struct timemono prev;
+	u64 num_channel_events;
 
 	/* Initialize wait indices: we're going to use it to generate ids. */
 	load_indexes(db, ld->indexes);
@@ -507,11 +508,13 @@ void migrate_from_account_db(struct lightningd *ld, struct db *db)
 
 	log_debug(ld->log, "Transferring %zu channel_events",
 		  tal_count(channel_events));
+
+	/* There can be lots of these, so do a single update at the end */
+	num_channel_events = 0;
 	for (size_t i = 0; i < tal_count(channel_events); i++) {
 		const struct channel_event *ev = channel_events[i];
 		struct mvt_account_id *account = tal(ev, struct mvt_account_id);
 		enum mvt_tag tag;
-		u64 id;
 
 		/* We removed currency support, because the only way you could
 		 * use it was to inject your own events, and nobody did that
@@ -536,8 +539,7 @@ void migrate_from_account_db(struct lightningd *ld, struct db *db)
 				 " payment_group_id,"
 				 " fees) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?);"));
 		set_mvt_account_id(account, NULL, ev->acct_name);
-		id = channel_mvt_index_created(ld, db, account, ev->credit, ev->debit);
-		db_bind_u64(stmt, id);
+		db_bind_u64(stmt, ++num_channel_events);
 		db_bind_mvt_account_id(stmt, db, account);
 		db_bind_credit_debit(stmt, ev->credit, ev->debit);
 		if (!mvt_tag_parse(ev->tag, strlen(ev->tag), &tag))
@@ -569,6 +571,12 @@ void migrate_from_account_db(struct lightningd *ld, struct db *db)
 			log_info(ld->log, "Inserted %zu/%zu channel_events", i, tal_count(channel_events));
 	}
 
+	wait_index_increase(ld, db,
+			    WAIT_SUBSYSTEM_CHANNELMOVES,
+			    WAIT_INDEX_CREATED,
+			    num_channel_events,
+			    NULL);
+
 	log_info(ld->log, "bookkeeper migration complete: migrated %zu chainmoves, %zu channelmoves, %zu descriptions",
 		 tal_count(chain_events),
 		 tal_count(channel_events),