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>

scripts/backfill_sidecars.py

@@ -300,12 +300,17 @@ def main(argv=None) -> int:
                 preserved_review     = None
                 preserved_ext        = None
                 preserved_bw_report  = None
+                preserved_txt_fn     = None
                 if sidecar_path.exists():
                     try:
                         _existing = event_file_io.read_sidecar(sidecar_path)
                         preserved_review    = _existing.get("review")
                         preserved_ext       = _existing.get("extensions")
                         preserved_bw_report = _existing.get("bw_report")
+                        # Preserve txt_filename so backfills don't blank out the
+                        # pointer to the saved raw .TXT (events ingested after
+                        # 2026-05-27 have this).
+                        preserved_txt_fn    = (_existing.get("source") or {}).get("txt_filename")
                     except Exception:
                         pass
 
@@ -334,6 +339,7 @@ def main(argv=None) -> int:
                     blastware_sha256=bw_sha,
                     source_kind=source_kind,
                     a5_pickle_filename=a5_filename,
+                    txt_filename=preserved_txt_fn,
                     review=preserved_review,
                     extensions=preserved_ext,
                 )