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_isr.py

@@ -142,7 +142,6 @@ class TestGetNextSplitTime:
         r._clock = fixed_clock(now)
         r.split_duration = split_minutes
         r.output_dir = cfg["output_directory"]
-        r.filename_pattern = "%Y%m%d_%H%M%S"
         r.max_retries = 3
         r.retry_delay = 1
         r.file_format = "auto"
@@ -184,7 +183,7 @@ class TestGetNextSplitTime:
 class TestGenerateFilename:
     """Tests for BaseRecorder.generate_filename()."""
 
-    def _recorder(self, pattern: str, now: datetime, output_dir: str) -> isr.BaseRecorder:
+    def _recorder(self, now: datetime, output_dir: str) -> isr.BaseRecorder:
         class _Rec(isr.BaseRecorder):
             def record(self): pass
 
@@ -198,28 +197,21 @@ class TestGenerateFilename:
         r._clock = fixed_clock(now)
         r.split_duration = 60
         r.output_dir = output_dir
-        r.filename_pattern = pattern
         r.max_retries = 3
         r.retry_delay = 1
         r.file_format = "auto"
         return r
 
-    def test_basic_pattern(self, tmp_path):
+    def test_fixed_format(self, tmp_path):
+        # Filenames are always <YYYYMMDD>_<HHMMSS>.<ext> — the recording start.
         now = datetime(2024, 12, 25, 14, 30, 0)
-        r = self._recorder("%Y%m%d_%H%M%S", now, str(tmp_path))
+        r = self._recorder(now, str(tmp_path))
         name = r.generate_filename("mp3")
         assert name.endswith("20241225_143000.mp3")
 
-    def test_subdirectory_created(self, tmp_path):
-        now = datetime(2024, 12, 25, 14, 30, 0)
-        r = self._recorder("%Y/%m/%d/rec_%H%M%S", now, str(tmp_path))
-        name = r.generate_filename("ogg")
-        parent = Path(name).parent
-        assert parent.exists()
-
     def test_output_dir_prefix(self, tmp_path):
         now = datetime(2024, 1, 1, 0, 0, 0)
-        r = self._recorder("%Y%m%d", now, str(tmp_path))
+        r = self._recorder(now, str(tmp_path))
         name = r.generate_filename("wav")
         assert name.startswith(str(tmp_path))
 
@@ -236,7 +228,6 @@ class TestDetectFormat:
             "url": "http://example.com/stream",
             "output_directory": "/tmp",
             "split_minutes": 60,
-            "filename_pattern": "%Y%m%d_%H%M%S",
             "max_retries": 1,
             "retry_delay_seconds": 0,
             "format": "auto",
@@ -252,7 +243,6 @@ class TestDetectFormat:
             r._clock = datetime.now
             r.split_duration = 60
             r.output_dir = "/tmp"
-            r.filename_pattern = "%Y%m%d_%H%M%S"
             r.max_retries = 1
             r.retry_delay = 0
             r.file_format = "auto"
@@ -446,7 +436,6 @@ class TestRecorderManagerLoadConfig:
 [general]
 output_directory = {str(tmp_path / "recordings")}
 split_minutes = 30
-filename_pattern = test_%Y%m%d
 max_retries = 3
 retry_delay_seconds = 2
 log_level = WARNING
@@ -491,7 +480,6 @@ format = ogg
 [general]
 output_directory = {str(tmp_path / "recordings")}
 split_minutes = 60
-filename_pattern = %Y%m%d_%H%M%S
 max_retries = 10
 retry_delay_seconds = 5
 log_level = WARNING
@@ -501,7 +489,6 @@ log_file = {log_file}
 type = stream
 url = http://example.com/stream
 split_minutes = 15
-filename_pattern = custom_%Y%m%d
 """
         cfg_path = tmp_path / "config.ini"
         cfg_path.write_text(config_text)
@@ -512,7 +499,6 @@ filename_pattern = custom_%Y%m%d
 
         rec = mgr.recorders[0]
         assert rec.split_duration == 15
-        assert rec.filename_pattern == "custom_%Y%m%d"
 
     def test_unknown_type_is_skipped(self, tmp_path):
         log_file = str(tmp_path / "test.log")
@@ -558,7 +544,6 @@ class TestStreamRecorderRecord:
             "url": "http://example.com/stream",
             "output_directory": "",  # overridden per-test with tmp_path
             "split_minutes": 60,
-            "filename_pattern": "%Y%m%d_%H%M%S",
             "max_retries": 2,
             "retry_delay_seconds": 0,
             "format": fmt,
@@ -610,7 +595,6 @@ class TestSoundcardRecorder:
             "format": "wav",
             "output_directory": str(tmp_path),
             "split_minutes": 60,
-            "filename_pattern": "%Y%m%d_%H%M%S",
             "max_retries": 1,
             "retry_delay_seconds": 0,
         }
@@ -639,7 +623,6 @@ class TestSoundcardRecorder:
             "format": "flac",
             "output_directory": str(tmp_path),
             "split_minutes": 60,
-            "filename_pattern": "%Y%m%d_%H%M%S",
             "max_retries": 1,
             "retry_delay_seconds": 0,
         }
@@ -662,7 +645,6 @@ class TestSoundcardRecorder:
             "format": "flac",
             "output_directory": str(tmp_path),
             "split_minutes": 60,
-            "filename_pattern": "%Y%m%d_%H%M%S",
             "max_retries": 1,
             "retry_delay_seconds": 0,
         }