feat: add /history switch for runtime database backend switching

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

src/command/cmd_defs.c

@@ -1882,17 +1882,20 @@ static const struct cmd_t command_defs[] = {
               CMD_TAG_CHAT)
       CMD_SYN(
               "/history on|off",
+              "/history switch sqlite|flatfile",
               "/history verify [<jid>]",
               "/history export [<jid>]",
               "/history import [<jid>]")
       CMD_DESC(
               "Switch chat history on or off, /logging chat will automatically be enabled when this setting is on. "
               "When history is enabled, previous messages are shown in chat windows. "
+              "Use 'switch' to change the active database backend at runtime without reconnecting. "
               "Use 'verify' to check integrity of stored message history. "
               "Use 'export' to copy messages from SQLite to flat-file format, or 'import' to copy from flat-file to SQLite. "
               "Both export and import merge with existing data (duplicates are skipped).")
       CMD_ARGS(
               { "on|off", "Enable or disable showing chat history." },
+              { "switch sqlite|flatfile", "Switch the active database backend at runtime." },
               { "verify [<jid>]", "Verify integrity of message history. Optionally specify a JID to check only one contact." },
               { "export [<jid>]", "Export SQLite history to flat-file format. Optionally specify a JID to export only one contact." },
               { "import [<jid>]", "Import flat-file history into SQLite. Optionally specify a JID to import only one contact." })