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/database.c

@@ -44,6 +44,7 @@
 #include "config.h"
 #include "database.h"
 #include "config/preferences.h"
+#include "config/accounts.h"
 #include "ui/ui.h"
 #include "xmpp/xmpp.h"
 #include "xmpp/message.h"
@@ -96,6 +97,34 @@ log_database_close(void)
     active_db_backend = NULL;
 }
 
+gboolean
+log_database_switch_backend(const char* new_backend)
+{
+    // Close current backend
+    log_database_close();
+
+    // Update preference
+    prefs_set_string(PREF_DBLOG, new_backend);
+    prefs_save();
+
+    // Get current account to reinitialize
+    const char* account_name = session_get_account_name();
+    if (!account_name) {
+        log_error("log_database_switch_backend: no active session");
+        return FALSE;
+    }
+
+    ProfAccount* account = accounts_get_account(account_name);
+    if (!account) {
+        log_error("log_database_switch_backend: account '%s' not found", account_name);
+        return FALSE;
+    }
+
+    gboolean result = log_database_init(account);
+    account_free(account);
+    return result;
+}
+
 void
 log_database_add_incoming(ProfMessage* message)
 {