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>

sfm/server.py

@@ -2178,6 +2178,39 @@ def db_event_blastware_file(event_id: str) -> FileResponse:
     )
 
 
+@app.get("/db/events/{event_id}/ascii_report.txt")
+def db_event_ascii_report_txt(event_id: str):
+    """Serve the raw BW ASCII report (.TXT) for an event, when preserved.
+
+    Returns 404 for events ingested before the .TXT-preservation feature
+    landed (2026-05-27) — those events have only the parsed ``bw_report``
+    block in the sidecar, not the raw .TXT.  Re-forwarding from the
+    watcher PC will populate the .TXT going forward.
+    """
+    row = _get_db().get_event(event_id)
+    if row is None:
+        raise HTTPException(status_code=404, detail=f"Event {event_id} not found")
+    serial   = row.get("serial")
+    filename = row.get("blastware_filename")
+    if not serial or not filename:
+        raise HTTPException(status_code=404, detail="Event has no associated BW file")
+    txt_path = _get_store().open_txt(serial, filename)
+    if txt_path is None:
+        raise HTTPException(
+            status_code=404,
+            detail=(
+                f"Raw .TXT not preserved for {filename}.  Events ingested "
+                "before 2026-05-27 don't have it; re-forward from the "
+                "watcher PC to populate."
+            ),
+        )
+    return FileResponse(
+        path=str(txt_path),
+        media_type="text/plain",
+        filename=txt_path.name,
+    )
+
+
 @app.get("/db/events/{event_id}/report.pdf")
 def db_event_report_pdf(event_id: str):
     """Render an Instantel-style Event Report as a PDF.