feat: minimum section duration filter (--min-duration, default 0.5 s)

A single 100 ms RMS window above the noise floor used to become its own
section, so isolated pops (clicks, single raindrops) flooded a day with
thousands of sub-second clips like "21:18 to 21:18". Sections shorter
than min_duration (measured after min_gap merging, so a cluster of blips
spanning longer still flags) are now discarded.

Wired through all coupled places: CLI flag, /api/config, controls-bar
input, /api/analyze query param, and the analysis-cache head keys (old
two-key caches no longer match and are recomputed on next analyse).

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

tests/test_web.py

@@ -7,9 +7,9 @@ from web import _loud_sections, _noise_floor_db
 WINDOW_DUR = 0.1  # 100 ms windows, as produced by WINDOW_SAMPLES at 48 kHz
 
 
-def _run(rms, margin_db=12.0, min_gap=2.0):
+def _run(rms, margin_db=12.0, min_gap=2.0, min_duration=0.5):
     duration = len(rms) * WINDOW_DUR
-    return _loud_sections(rms, WINDOW_DUR, duration, margin_db, min_gap)
+    return _loud_sections(rms, WINDOW_DUR, duration, margin_db, min_gap, min_duration)
 
 
 def test_burst_above_quiet_floor_is_detected():
@@ -60,6 +60,38 @@ def test_min_gap_merges_nearby_bursts():
     assert sections[1]['start'] == 90.0
 
 
+def test_min_duration_drops_subsecond_blips():
+    # Isolated single-window pops (clicks, single raindrops) spaced wider than
+    # min_gap must not each become their own section — this is what used to
+    # produce thousands of zero-length sections per day.
+    rms = [0.002] * 1200
+    for i in range(600, 660, 30):   # 0.1 s blips, 3 s apart (> min_gap)
+        rms[i] = 0.05
+    assert _run(rms) == []
+    # With the filter disabled they are all reported
+    assert len(_run(rms, min_duration=0.0)) == 2
+
+
+def test_min_duration_keeps_sections_at_or_above_it():
+    rms = [0.002] * 1200
+    rms[600:605] = [0.05] * 5     # exactly 0.5 s
+    sections = _run(rms, min_duration=0.5)
+    assert len(sections) == 1
+    assert sections[0]['start'] == 60.0
+
+
+def test_min_duration_applies_after_gap_merging():
+    # Two sub-min_duration blips within min_gap merge into one section whose
+    # loud span exceeds min_duration — the merged section must survive.
+    rms = [0.002] * 1200
+    rms[600] = 0.05
+    rms[610] = 0.05               # 1 s apart < 2 s min_gap → merged, 1.1 s span
+    sections = _run(rms, min_duration=1.0)
+    assert len(sections) == 1
+    assert sections[0]['start'] == 60.0
+    assert sections[0]['end'] >= 61.0
+
+
 def test_noise_floor_tracks_blocks_and_ignores_short_events():
     quiet_db = 20 * math.log10(0.002)
     db = [quiet_db] * 1200