fix(log): roll daily log file at midnight for long-running processes

The log filename was computed once at init_logging() startup and the
RotatingFileHandler only rotated by size, so a process running across
midnight kept appending every subsequent day's logs to the start-day
file (langbot-<start date>.log). No file ever appeared for the current
day until the process was restarted, confusing users into thinking
logging had stopped.

Replace RotatingFileHandler with DailyGroupedRotatingFileHandler, which
switches to langbot-<current date>.log when the local date changes while
still doing size-based numbered rotation within a day. On-disk naming
stays compatible with the maintenance log-retention cleanup
(LOG_FILE_PATTERN). Adds regression tests.

tests/unit_tests/core/test_bootutils_log.py

@@ -0,0 +1,120 @@
+"""Tests for the daily-grouped rotating log file handler.
+
+Regression coverage for the bug where a long-running process names its log
+file after the *start* day and keeps appending to it across midnight, so no
+file ever appears for the current day. See
+``langbot.pkg.core.bootutils.log.DailyGroupedRotatingFileHandler``.
+"""
+
+from __future__ import annotations
+
+import logging
+import os
+import re
+
+import langbot.pkg.core.bootutils.log as logmod
+from langbot.pkg.core.bootutils.log import DailyGroupedRotatingFileHandler
+
+# Mirror of the cleanup pattern in api/http/service/maintenance.py.
+MAINTENANCE_LOG_FILE_PATTERN = re.compile(r'^langbot-(\d{4}-\d{2}-\d{2})\.log(?:\.\d+)?$')
+
+
+def _listing(directory):
+    return sorted(os.listdir(directory))
+
+
+def _make_logger(handler, name):
+    logger = logging.getLogger(name)
+    logger.setLevel(logging.INFO)
+    logger.handlers.clear()
+    logger.addHandler(handler)
+    logger.propagate = False
+    return logger
+
+
+class TestDailyGroupedRotatingFileHandler:
+    def _patch_date(self, monkeypatch, box):
+        """Make the handler read its current date from ``box['date']``."""
+
+        def fake_strftime(fmt, t=None):
+            if fmt == '%Y-%m-%d':
+                return box['date']
+            return '00:00:00'
+
+        monkeypatch.setattr(logmod.time, 'strftime', fake_strftime)
+
+    def test_initial_file_named_for_current_day(self, tmp_path, monkeypatch):
+        box = {'date': '2026-06-08'}
+        self._patch_date(monkeypatch, box)
+
+        handler = DailyGroupedRotatingFileHandler(str(tmp_path), max_bytes=10_000, backup_count=3)
+        logger = _make_logger(handler, 'lb_logtest_initial')
+        logger.info('hello')
+        handler.close()
+
+        assert _listing(tmp_path) == ['langbot-2026-06-08.log']
+
+    def test_same_day_size_rotation_creates_numbered_backups(self, tmp_path, monkeypatch):
+        box = {'date': '2026-06-08'}
+        self._patch_date(monkeypatch, box)
+
+        handler = DailyGroupedRotatingFileHandler(str(tmp_path), max_bytes=200, backup_count=3)
+        logger = _make_logger(handler, 'lb_logtest_size')
+        for i in range(40):
+            logger.info('padding line to exceed maxBytes %d', i)
+        handler.close()
+
+        files = _listing(tmp_path)
+        assert 'langbot-2026-06-08.log' in files
+        assert any(f.startswith('langbot-2026-06-08.log.') for f in files)
+
+    def test_rolls_to_new_file_when_day_changes(self, tmp_path, monkeypatch):
+        box = {'date': '2026-06-08'}
+        self._patch_date(monkeypatch, box)
+
+        handler = DailyGroupedRotatingFileHandler(str(tmp_path), max_bytes=10_000, backup_count=3)
+        logger = _make_logger(handler, 'lb_logtest_midnight')
+        logger.info('day1 line')
+
+        # Simulate crossing midnight within the same running process.
+        box['date'] = '2026-06-09'
+        logger.info('day2 line after midnight')
+        handler.close()
+
+        files = _listing(tmp_path)
+        assert 'langbot-2026-06-08.log' in files
+        assert 'langbot-2026-06-09.log' in files
+
+        day2 = (tmp_path / 'langbot-2026-06-09.log').read_text(encoding='utf-8')
+        assert 'day2 line after midnight' in day2
+        assert 'day1 line' not in day2
+
+    def test_rollover_repeats_across_multiple_days(self, tmp_path, monkeypatch):
+        box = {'date': '2026-06-08'}
+        self._patch_date(monkeypatch, box)
+
+        handler = DailyGroupedRotatingFileHandler(str(tmp_path), max_bytes=10_000, backup_count=3)
+        logger = _make_logger(handler, 'lb_logtest_multiday')
+        for day in ('2026-06-08', '2026-06-09', '2026-06-10'):
+            box['date'] = day
+            logger.info('line for %s', day)
+        handler.close()
+
+        files = _listing(tmp_path)
+        for day in ('2026-06-08', '2026-06-09', '2026-06-10'):
+            assert f'langbot-{day}.log' in files
+
+    def test_all_filenames_match_maintenance_cleanup_pattern(self, tmp_path, monkeypatch):
+        box = {'date': '2026-06-08'}
+        self._patch_date(monkeypatch, box)
+
+        handler = DailyGroupedRotatingFileHandler(str(tmp_path), max_bytes=200, backup_count=3)
+        logger = _make_logger(handler, 'lb_logtest_pattern')
+        for i in range(40):
+            logger.info('padding line %d', i)
+        box['date'] = '2026-06-09'
+        logger.info('next day line')
+        handler.close()
+
+        for name in _listing(tmp_path):
+            assert MAINTENANCE_LOG_FILE_PATTERN.match(name), name