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_export.c

@@ -213,7 +213,6 @@ log_database_export_to_flatfile(const gchar* const contact_jid)
     if (!g_flatfile_account_jid) {
         // Flatfile backend not initialized — init it temporarily
         // We need g_flatfile_account_jid for path construction
-        g_free(g_flatfile_account_jid);
         g_flatfile_account_jid = g_strdup(myjid->barejid);
     }
 
@@ -289,7 +288,9 @@ log_database_export_to_flatfile(const gchar* const contact_jid)
 
         // Lock the temp file
         int fd = fileno(fp);
-        flock(fd, LOCK_EX);
+        if (flock(fd, LOCK_EX) != 0) {
+            log_warning("export: flock(%s) failed: %s", tmp_path, strerror(errno));
+        }
 
         fprintf(fp, "%s", FLATFILE_HEADER);
 
@@ -416,7 +417,6 @@ log_database_import_from_flatfile(const gchar* const contact_jid)
     }
 
     if (!g_flatfile_account_jid) {
-        g_free(g_flatfile_account_jid);
         g_flatfile_account_jid = g_strdup(myjid->barejid);
     }
 
@@ -522,6 +522,13 @@ log_database_import_from_flatfile(const gchar* const contact_jid)
 
             sqlite_be->add_incoming(msg);
             message_free(msg);
+
+            // Verify the row was actually inserted
+            if (db_sqlite_last_changes() < 1) {
+                log_error("import: insert failed for %s at %s", contact, ts);
+                import_ok = FALSE;
+                break;
+            }
             contact_imported++;
 
             if (contact_imported % 500 == 0) {