ingest: preserve raw BW ASCII report (.TXT) alongside the binary

Previously the .TXT was parsed into the sidecar's bw_report projection
and then discarded at ingest time.  Now save_imported_bw() writes it
to <store>/<serial>/<filename>_ASCII.TXT permanently.

Rationale: with BW Mail / Forwarding Agent being phased out of the
operator workflow, the XML/PDF/WMF those tools produce won't be
available — the binary + .TXT (created by BW ACH itself) are our
only authoritative inputs going forward.  Keeping the raw .TXT
unlocks:

  - Parser bug fixes can be applied RETROACTIVELY by re-parsing the
    stored .TXT, instead of requiring a re-forward from the watcher
    PC (which lost the .TXT after BW ACH cleanup).
  - Audit trail of what BW actually sent us, for debugging.
  - The five known parser-PPV-miss events will be re-parseable once
    the regex fix lands (instead of staying broken indefinitely).

Storage cost: ~15 KB per event × 14k events = ~210 MB on the
existing prod corpus.  Negligible.

Implementation:
  - WaveformStore gains txt_path_for() + open_txt()
  - save_imported_bw() writes the .TXT when bw_report_text is supplied
  - sidecar source block records the txt_filename
  - backfill_sidecars.py preserves txt_filename across regens
  - New GET /db/events/{id}/ascii_report.txt endpoint serves it
  - Returns 404 for events ingested before this change (no .TXT in
    the store yet) — re-forward to populate

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

minimateplus/event_file_io.py

@@ -332,6 +332,7 @@ def event_to_sidecar_dict(
     blastware_filesize: int,
     blastware_sha256: str,
     source_kind: str = "sfm-live",
+    txt_filename: Optional[str] = None,
     a5_pickle_filename: Optional[str] = None,
     tool_version: str = _TOOL_VERSION_DEFAULT,
     captured_at: Optional[datetime.datetime] = None,
@@ -448,6 +449,7 @@ def event_to_sidecar_dict(
             "captured_at":        captured_at.isoformat() + "Z" if captured_at.tzinfo is None else captured_at.isoformat(),
             "tool_version":       tool_version,
             "a5_pickle_filename": a5_pickle_filename,
+            "txt_filename":       txt_filename,
         },
 
         "review": review or {