Adds check_and_clear_idb_tables() helper in core__idb_dexie.ts that clears
Dexie tables when their content version changes. Version numbers live in
IDB_CONTENT_VERSIONS (store_versions.ts); state tracked in localStorage
(ae_idb_ver__{module}__{table}) so each table clears exactly once per bump.
Wired into db_journals.ts (pilot): journal_entry at v3 clears stale
content_md_html/history_md_html data cached before the properties_to_save fix.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>