Import the longest bank-permitted history and surface real provider errors

Manual history imports failed opaquely once a bank capped lookback on an
established consent (N26 rejects date_from beyond ~90 days with
WRONG_TRANSACTIONS_PERIOD). Backfill now requests the documented longest
fetching strategy, reports the coverage the bank actually provided, and
non-2xx responses surface allowlisted documented error codes instead of a
generic fallback. Dead-session codes map to reconnection. Failed syncs
retry hourly so a stale sync banner no longer persists for a day.

web/src/Accounts.tsx

@@ -441,18 +441,28 @@ function BackfillHistory({
           setError("");
           setSuccess("");
           try {
-            const response = await request<{ imported: number; state: State }>(
-              "/api/backfill",
-              {
-                revision: state.revision,
-                account_id: account.id,
-                history_months: months,
-              },
-            );
-            const message =
+            const response = await request<{
+              imported: number;
+              requested_from?: string;
+              earliest_fetched?: string;
+              state: State;
+            }>("/api/backfill", {
+              revision: state.revision,
+              account_id: account.id,
+              history_months: months,
+            });
+            let message =
               response.imported === 0
                 ? `No new transactions imported for ${account.display_name}. Existing transactions were not duplicated.`
                 : `Imported ${response.imported} new transactions for ${account.display_name}. Existing transactions were not duplicated.`;
+            if (!response.earliest_fetched) {
+              message += " The bank returned no transactions for this range.";
+            } else if (
+              response.requested_from &&
+              response.earliest_fetched > response.requested_from
+            ) {
+              message += ` The bank provided history starting ${response.earliest_fetched}, not the requested ${response.requested_from}; banks often limit how far back an existing connection can read. Older transactions can be added via CSV import.`;
+            }
             acceptState(response.state, message);
             setSuccess(message);
           } catch (err) {