perf: refresh exchange rates in one transaction with one prepared statement

[thorstenhornung1]

Builds on #1165 and #1166. If you would prefer this standalone, I will rebase it onto main.

The problem

Both rate refresh paths — endpoints/cronjobs/updateexchange.php and endpoints/currency/update_exchange.php — do this:

foreach ($apiData['rates'] as $currencyCode => $rate) {
    ...
    $updateStmt = $db->prepare($updateQuery);   // re-prepared every iteration
    ...
    $updateResult = $updateStmt->execute();     // and autocommitted on its own

    if (!$updateResult) {
        echo "Error updating rate for currency: $currencyCode";   // then carries on
    }
}

Two problems follow.

A partial refresh is silently wrong. Rates are only comparable when they share a conversion base. If the loop stops halfway — a truncated provider response, a failing write, a fatal error — some currencies are converted against the new base and some against the old one. Totals still render, they are just incorrect, and nothing indicates it.

The same statement is parsed once per currency. It never changes between iterations.

The change

• prepare the update once, before the loop, and reuse it with reset()
• wrap one user's rate writes and their refresh timestamp in a transaction
• roll back and report when a write fails, instead of continuing with a half-converted set

The cron job keeps its per-user granularity: one user's failure does not affect the users refreshed before or after them.

Verification

New test cases prove the semantics rather than the implementation:

• an interrupted refresh leaves the previous rates intact
• a completed refresh commits every rate together
• five writes need one prepare
• a structural guard that both refresh paths open, commit and roll back a transaction, and that the prepare sits outside the loop — so a future path that autocommits per currency fails the tests

Also ran the cron job against a seeded database with six users and confirmed it leaves no transaction open when the provider is unreachable.