fix(backfill): clear stale shape_* when the .h5 can no longer yield a shape

backfill_event_shape.py skipped rows whose .h5 produced no shape and left
the previously stored value in place.  A stale shape outlives the decode
it came from and silently feeds the false-trigger detector.

Found while re-running the backfill after the histogram codec fix: 493
rows in the prod snapshot were carrying shape metrics that no longer
matched their .h5 — e.g. BE17353/S353LDOK.XZ0H held crest_factor from a
223-sample decode while its .h5 holds a single interval.  These predate
today's work (present in the pre-32000 snapshot), so this is pre-existing
behaviour rather than fallout from the codec fixes.

Now NULLs shape_crest_factor / shape_near_peak_count / shape_sample_count
/ shape_axis in that case and reports a `cleared_stale` count.  Verified
on the snapshot: 493 cleared, 0 stale rows remaining, 11570 rows matching
their .h5 exactly.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgTe8CamXAHcAmaQ6QNcog

scripts/backfill_event_shape.py

@@ -11,7 +11,8 @@ from sfm.shape_metrics import shape_from_h5
 log = logging.getLogger("backfill_event_shape")
 
 def backfill_shape(db: SeismoDb, store: WaveformStore, *, dry_run: bool = False) -> dict:
-    counts = {"updated": 0, "skipped_no_h5": 0, "skipped_no_samples": 0}
+    counts = {"updated": 0, "skipped_no_h5": 0, "skipped_no_samples": 0,
+              "cleared_stale": 0}
     for row in db.query_events(limit=1_000_000):
         serial, filename = row.get("serial"), row.get("blastware_filename")
         if not serial or not filename:
@@ -21,6 +22,20 @@ def backfill_shape(db: SeismoDb, store: WaveformStore, *, dry_run: bool = False)
             counts["skipped_no_h5"] += 1; continue
         shape = shape_from_h5(h5_path)
         if shape is None:
+            # The .h5 can no longer yield a shape (fewer than 2 samples, or a
+            # flat trace).  Clear any previously stored value rather than
+            # leaving it behind — a stale shape outlives the decode it came
+            # from and silently feeds the false-trigger detector.  Seen after
+            # a decoder fix shrinks an event: 493 rows in the prod snapshot
+            # were carrying metrics from a superseded decode (2026-08-25).
+            if row.get("shape_crest_factor") is not None:
+                if not dry_run:
+                    with db._connect() as conn:
+                        conn.execute(
+                            "UPDATE events SET shape_crest_factor=NULL, "
+                            "shape_near_peak_count=NULL, shape_sample_count=NULL, "
+                            "shape_axis=NULL WHERE id=?", (row["id"],))
+                counts["cleared_stale"] += 1
             counts["skipped_no_samples"] += 1; continue
         if not dry_run:
             with db._connect() as conn: