Add log_database_switch_backend() that closes the current backend,
updates PREF_DBLOG preference, and reinitializes with the new backend
without requiring a reconnect or restart.
New command: /history switch sqlite|flatfile
- Validates connection state and backend name
- Skips if already using the requested backend
- Autocomplete support for the subcommand
jabber.developer2
b5fe714a89
