* fix(sync): don't persist zero balances when a provider balance fetch fails
A nil current_balance means the sync's balance fetch did not succeed
(the snapshot upsert clears it; only a successful fetch repopulates it).
The Lunchflow and Enable Banking processors coerced nil to 0 and write
it (plus a currency fallback) onto the linked account, so any transient
provider failure persists wrong data with no user-visible signal.
Instead, treat nil as no-data: skip the account update. Also stop the
Lunchflow snapshot upsert resetting an established account's currency to
USD when the accounts endpoint omits currency.
* fix(lunchflow): normalize the preserved currency in snapshot upsert
Addresses the review comment on #2617: the preserved fallback reused the
record's raw in-memory currency, so a blank value would fail the presence
validation and break import, and an invalid code would persist instead of
falling back to USD. Run it through parse_currency like the payload value.
Regression test added.
* fix(lunchflow,eb): review round 2 — failure visibility and currency parity
Addresses the three review findings on #2617:
1. Capture the Lunch Flow balance-fetch failure via DebugLogEntry (the sync
otherwise reports success with no mention of the skip).
This is Lunch Flow only: Enable Banking's importer already surfaces the
failure through transactions_failed/@sync_error.
2. Apply the currency-preservation fix to Enable Banking's snapshot upsert
(same shape as the Lunch Flow fix: parity/safety). Regression test added.
3. Label the Enable Banking processor currency assertion as a parity check —
it also passes on main, since the reset defect was Lunch Flow-specific.
