feat: name cut clips by wall-clock time; fix recording filename format

Cut downloads were named by byte offsets (`..._cut_740s-750s.flac`). They are
now named by the actual recording time the slice covers, e.g.
`20260523_22-31-30_22-32-30.flac` for a 22:31:30->22:32:30 cut of a recording
started at 22:00:00.

To make this reliable, the recording filename is now a fixed
`%Y%m%d_%H%M%S` start-time format (`FILENAME_FORMAT`) shared by isr.py and
web.py, replacing the user-configurable `filename_pattern` (web.py never reads
config.ini, so a custom pattern could not be parsed back). web.py parses the
start time out of the filename via `_recording_start()` and builds cut names
with `_cut_filename()`. The DATE column now also comes from the filename
(falling back to mtime only for non-standard names), since mtime is the last
write, not the start.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_web.py

@@ -2,7 +2,7 @@
 
 import math
 
-from web import _loud_sections, _noise_floor_db
+from web import _cut_filename, _loud_sections, _noise_floor_db, _recording_start
 
 WINDOW_DUR = 0.1  # 100 ms windows, as produced by WINDOW_SAMPLES at 48 kHz
 
@@ -99,3 +99,33 @@ def test_noise_floor_tracks_blocks_and_ignores_short_events():
     floor = _noise_floor_db(db, WINDOW_DUR)
     assert len(floor) == len(db)
     assert all(abs(f - quiet_db) < 1.0 for f in floor)
+
+
+# ---------------------------------------------------------------------------
+# Filename parsing / cut naming
+# ---------------------------------------------------------------------------
+
+def test_recording_start_parses_standard_name():
+    from datetime import datetime
+    assert _recording_start("20260523_220000") == datetime(2026, 5, 23, 22, 0, 0)
+
+
+def test_recording_start_rejects_nonstandard_name():
+    assert _recording_start("radio1_20260523") is None
+    assert _recording_start("notes") is None
+
+
+def test_cut_filename_uses_wall_clock_span():
+    # Recording started 22:00:00; cut covers 22:31:30 → 22:32:30.
+    name = _cut_filename("20260523_220000", ".flac", 1890, 1950)
+    assert name == "20260523_22-31-30_22-32-30.flac"
+
+
+def test_cut_filename_rolls_over_the_hour():
+    name = _cut_filename("20260523_220000", ".wav", 3590, 3661)
+    assert name == "20260523_22-59-50_23-01-01.wav"
+
+
+def test_cut_filename_falls_back_for_nonstandard_name():
+    name = _cut_filename("mixtape", ".mp3", 740, 750.4)
+    assert name == "mixtape_cut_740s-750s.mp3"