delete: also clean up preserved _ASCII.TXT file

_cleanup_event_files() removes the on-disk artifacts when an event is
hard-deleted (binary, a5_pickle, sidecar, h5).  Today's .TXT
preservation feature added a new on-disk file (_ASCII.TXT next to the
binary) but the cleanup didn't know about it — so any event deleted
via /db/events/{id} (single) or /db/events/delete_bulk (or the
Terra-View "SFM Event DB Manager" UI which proxies through to those
endpoints) was leaving orphan .TXT files in the store.

Added "txt" to the cleanup list using the new
WaveformStore.txt_path_for().  Safe for old events without a .TXT —
the exists() check skips the unlink.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sfm/server.py

@@ -1987,10 +1987,15 @@ def _cleanup_event_files(row: dict) -> dict:
     base_name = bw_name or a5_name or sc_name
     if base_name:
         bw_path, a5_path = store.paths_for(serial, base_name)
-        sc_path = store.sidecar_path_for(serial, base_name)
-        h5_path = store.hdf5_path_for(serial, base_name)
+        sc_path  = store.sidecar_path_for(serial, base_name)
+        h5_path  = store.hdf5_path_for(serial, base_name)
+        # Preserved BW ASCII report (added 2026-05-27 with the .TXT
+        # preservation feature) — needs to be cleaned up too, otherwise
+        # deletes leave orphan _ASCII.TXT files behind.
+        txt_path = store.txt_path_for(serial, base_name)
         for kind, p in [("blastware", bw_path), ("a5_pickle", a5_path),
-                        ("sidecar", sc_path), ("hdf5", h5_path)]:
+                        ("sidecar", sc_path), ("hdf5", h5_path),
+                        ("txt", txt_path)]:
             try:
                 if p.exists():
                     p.unlink()