Don't be strict on serials when rebuilding snapshot

It seems that we have at least one board where the stored history is
missing exactly one event (between two item.front events) and in this
case it's better to try to patch it up than to give up. The root cause
should of course be fixed as well.

backend/src/board-store.ts

@@ -50,16 +50,17 @@ export async function fetchBoard(id: Id): Promise<BoardAndAccessTokens | null> {
             let board = snapshot
 
             let i = 0
+            let rebuildingSnapshot = false
             function updateBoardWithEventChunk(chunk: BoardHistoryEntry[]) {
                 board = chunk.reduce((b, e) => {
                     i++
                     if (e.action === "board.setAccessPolicy") {
                         // Don't process access policy event when fetching board
                         // Access policy may have been changed in the database after the event
-                        // And the database status is considered the master
+                        // And the board table status is considered the master
                         return b
                     }
-                    return boardReducer(b, e, { inplace: true, strictOnSerials: true })[0]
+                    return boardReducer(b, e, { inplace: true, strictOnSerials: !rebuildingSnapshot })[0]
                 }, board)
                 historyEventCount += chunk.length
                 lastSerial = chunk[chunk.length - 1].serial ?? snapshot.serial
@@ -68,10 +69,11 @@ export async function fetchBoard(id: Id): Promise<BoardAndAccessTokens | null> {
             await getBoardHistory(id, snapshot.serial, updateBoardWithEventChunk).catch(async (error) => {
                 console.error(error.message)
                 console.error(
-                    `Error applying board history for snapshot update for board ${id}. Loop index ${i}. Rebooting snapshot...`,
+                    `Error applying board history for snapshot update for board ${id}. Loop index ${i}. Rebooting snapshot. This may be a lossy operation.`,
                 )
                 i = 0
                 board = { ...snapshot, items: {}, connections: [] }
+                rebuildingSnapshot = true
                 try {
                     await getFullBoardHistory(id, client, updateBoardWithEventChunk)
                 } catch (e) {