feat: onset-aware section scoring so slow swells rank at the bottom

A section's score is now its peak dB above the noise floor capped by
the sharpest rise within ONSET_SECONDS (0.5 s). Real events (voices,
impacts, barks) rise fast and keep their full prominence; a gradual
swell that outruns the 30 s floor blocks (gusts, distant approaching
cars) still flags but scores near zero, so score-ranked review (chips,
U/I highlights, "Highlights only" mode) surfaces events first. A
section starting in a file's first 0.5 s is scored against the floor
instead, so events cut off by a file split are not punished as swells.

Old cached analyses carry now-wrong scores, so the cache gains a
leading "detector" version key (DETECTOR_VERSION = 2) checked by both
_cached_analysis_params() and the /api/analyze cache hit path; v1
caches never match and are recomputed on the next analyse.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_web.py

@@ -92,6 +92,37 @@ def test_min_duration_applies_after_gap_merging():
     assert sections[0]['end'] >= 61.0
 
 
+def test_slow_swell_scores_near_zero_sharp_burst_scores_high():
+    # A ~10 s swell rises faster than the 30 s floor blocks can track, so it
+    # still flags — but its score must collapse (onset cap) so it ranks at the
+    # bottom, while a sharp burst with the same peak keeps its full score.
+    rms = [0.002] * 1200
+    for k in range(80):                       # rise −54 → −28 dB over 8 s
+        rms[400 + k] = 0.002 * 10 ** (26 * (k / 80) / 20)
+    rms[480:500] = [0.04] * 20                # hold 2 s at −28 dB
+    for k in range(80):                       # fall back over 8 s
+        rms[500 + k] = 0.04 * 10 ** (-26 * (k / 80) / 20)
+    rms[900:910] = [0.04] * 10                # sharp 1 s burst, same peak
+    sections = _run(rms)
+    assert len(sections) == 2
+    swell, burst = sections
+    assert swell['start'] < 50.0 < 90.0 <= burst['start']
+    assert burst['score'] >= 24.0             # ≈ full 26 dB prominence
+    assert swell['score'] <= 5.0              # capped by its slow onset
+    assert swell['score'] < burst['score']
+
+
+def test_burst_at_file_start_keeps_full_score():
+    # No pre-event history to measure a rise against: the onset cap falls back
+    # to prominence above the floor, so events cut off by a file split are not
+    # punished as swells.
+    rms = [0.05] * 10 + [0.002] * 1190
+    sections = _run(rms)
+    assert len(sections) == 1
+    assert sections[0]['start'] == 0.0
+    assert sections[0]['score'] >= 24.0
+
+
 def test_noise_floor_tracks_blocks_and_ignores_short_events():
     quiet_db = 20 * math.log10(0.002)
     db = [quiet_db] * 1200