Synchronize connected accounts twice a day

One named syncInterval replaces the two 24-hour literals in the scheduler: a
successful run now waits 12 hours, and an account becomes due again 12 hours
after its last successful sync. Two background fetches a day stay inside Enable
Banking's documented allowance of roughly four per account per day, which a
failing sync's hourly retries also draw from, so the bank rate limits that
prompted this are no more likely than before.

README.md

@@ -142,7 +142,7 @@ Open **Accounts → Connect your bank**:
 
 Finance Duck verifies the callback state, exchanges the returned code for a `session_id`, stores the session locally with restrictive permissions, and wakes the synchronization worker immediately. You do not need to copy the code or session ID manually.
 
-Initial synchronization requests the selected number of **calendar months of booked transactions per account**, defaulting to **12 months**. The bank may provide less history. The choice is saved with the bank connection and reused on reconnection. Subsequent daily synchronization overlaps each account's last successful sync by **14 days**. **Sync now** starts a manual synchronization. Existing accounts keep their successful-sync cursors: changing the history choice or reconnecting does **not** backfill them. Older history can be imported with CSV.
+Initial synchronization requests the selected number of **calendar months of booked transactions per account**, defaulting to **12 months**. The bank may provide less history. The choice is saved with the bank connection and reused on reconnection. Automatic synchronization then runs **twice a day**, every **12 hours** after the last successful run, overlapping each account's last successful sync by **14 days**. **Sync now** starts a manual synchronization at any time. Existing accounts keep their successful-sync cursors: changing the history choice or reconnecting does **not** backfill them. Older history can be imported with CSV.
 
 **HTTP 429 is a provider rate limit, not evidence that bank consent has expired.** Bank reads honor `Retry-After` and use bounded exponential retries. A longer or exhausted limit pauses further requests until the reported retry time; failed accounts keep their previous sync cursors and imported data. Session checks use the saved account metadata rather than fetching every account's details again. A failed session is reported once instead of also marking each of its accounts unavailable. After the cooldown, **Sync now** can retry; the warning clears after a successful sync. One-time authorization and code-exchange requests are never automatically replayed.