perf: O(1) dedup/LMC cache, g_slist_append → prepend+reverse

- Add archive_ids hash set and stanza_senders hash map to
  ff_contact_state_t, populated during index build/extend via
  lightweight _ff_cache_line_ids() (no full parse overhead).
- Replace O(n) file scans in _ff_has_archive_id() and
  _ff_find_original_sender() with O(1) hash table lookups.
- Replace g_slist_append with g_slist_prepend + g_slist_reverse
  in _ff_apply_lmc, _flatfile_get_previous_chat, and
  ff_verify_integrity to avoid O(n²) list building.
- Add db_sqlite_last_changes() declaration to database.h.

src/database_sqlite.c

@@ -860,6 +860,14 @@ db_sqlite_rollback_transaction(void)
     }
 }
 
+int
+db_sqlite_last_changes(void)
+{
+    if (!g_chatlog_database)
+        return 0;
+    return sqlite3_changes(g_chatlog_database);
+}
+
 GSList*
 db_sqlite_get_all_chat(const gchar* const contact_barejid)
 {
@@ -949,10 +957,10 @@ db_sqlite_list_contacts(void)
     while (sqlite3_step(stmt) == SQLITE_ROW) {
         const char* jid = (const char*)sqlite3_column_text(stmt, 0);
         if (jid && strlen(jid) > 0) {
-            contacts = g_slist_append(contacts, g_strdup(jid));
+            contacts = g_slist_prepend(contacts, g_strdup(jid));
         }
     }
     sqlite3_finalize(stmt);
 
-    return contacts;
+    return g_slist_reverse(contacts);
 }