* fix(snaptrade): import non-primary-currency cash as cash holdings
Fixes#1809.
SnaptradeAccount#upsert_balances! picked a single cash entry (account
currency -> USD -> first) and stored only that in cash_balance; every
other currency's cash was discarded. A moomoo Canada account with CAD
$500 + USD $1000 imported only the CAD.
Persist the full balances snapshot (new raw_balances_payload column) and
surface each non-primary-currency cash entry as a synthetic per-currency
cash holding (Security.cash_for(account, currency:)), mirroring the
existing cash-security pattern. The primary currency stays in
cash_balance. HoldingsProcessor now also runs for cash-only balances, and
the Processor invokes it when there are holdings OR non-primary cash.
Cash holdings use a stable external_id so repeated syncs update rather
than duplicate.
* fix(snaptrade): encrypt raw_balances_payload and drop cash amount from log
Addresses PR #1979 review: Codex P1 (encrypt the newly persisted balances snapshot at rest, matching the other raw provider payloads) and CodeRabbit nitpick (do not log monetary amounts at info level).
* refactor(snaptrade): extract primary_cash_entry and harden balances test
PR #1979 review: extract the shared account-currency->USD->first cash selection into a private helper (CodeRabbit DRY nitpick); reorder the upsert_balances! test so the primary currency is not first, proving dig(:currency,:code) resolves it on string-keyed payloads rather than the entries.first fallback (jjmata).
