fix(dedup): scope content hashes by account

[smitzo]

Summary

• scope fallback content hashes by account_id when one is available
• preserve the existing hash format for transactions without an account ID
• cover both same-account deduplication and cross-account preservation
• update the documented fallback identity fields

Root cause

FITID-based deduplication already keys transactions by (account_id, fitid), but the fallback content hash used only date, signed amount, currency, and canonical description. Two transactions from different accounts could therefore receive the same fallback identity and one would be silently discarded during a multi-statement merge.

For example, matching MT940 entries from ACCOUNT-A and ACCOUNT-B previously merged into one row even though both source statements parsed successfully.

Impact

Identical-looking transactions from different known accounts are now preserved. Transactions from the same account continue to deduplicate normally.

Compatibility

The account ID is added to the hash only when present, so existing hashes for CSV, QIF, text, or other account-less transactions remain unchanged. Hashes for account-aware transactions intentionally become account-scoped.