update() and update_fields() lack transaction for memory + tags dual-write

[ajianaz]

Problem

Same bug class as the fixed insert() (PR #970), but in two more methods:

1. crud.rs:243 — update() does UPDATE memories then DELETE/INSERT tags without transaction
2. crud.rs:295 — update_fields() same pattern

If tag insert fails after memory UPDATE, tags are left in inconsistent state (deleted but not re-inserted).

Fix

Wrap both methods in unchecked_transaction() — same pattern as the fixed insert().

Severity: MAJOR (data integrity)

[ajianaz]

Fixed in #983 (merged to develop)