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

tests/unittests/database/stub_database.c

@@ -25,9 +25,14 @@
 
 #include "config.h"
 #include "database.h"
+#include "database_flatfile.h"
 
 db_backend_t* active_db_backend = NULL;
 
+// The flatfile parser needs this global for path construction.
+// In tests we keep it NULL since no real file I/O happens via the backend.
+char* g_flatfile_account_jid = NULL;
+
 gboolean
 log_database_init(ProfAccount* account)
 {
@@ -53,6 +58,11 @@ void
 log_database_close(void)
 {
 }
+gboolean
+log_database_switch_backend(const char* new_backend)
+{
+    return TRUE;
+}
 GSList*
 log_database_verify_integrity(const gchar* const contact_barejid)
 {