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.

src/langbot/pkg/core/bootutils/log.py

@@ -1,5 +1,6 @@
 import logging
 import logging.handlers
+import os
 import sys
 import time
 
@@ -20,6 +21,66 @@ log_colors_config = {
 LOG_FILE_MAX_BYTES = 10 * 1024 * 1024  # 10MB per file
 LOG_FILE_BACKUP_COUNT = 5  # Keep 5 backup files (total ~50MB max)
 
+LOG_DIR = 'data/logs'
+
+
+class DailyGroupedRotatingFileHandler(logging.handlers.RotatingFileHandler):
+    """File handler that writes to ``data/logs/langbot-YYYY-MM-DD.log``.
+
+    It combines two rotation triggers:
+
+    * **Size** — within a single day the file is rotated once it exceeds
+      ``maxBytes``, producing numbered backups (``langbot-DATE.log.1`` etc.),
+      exactly like :class:`~logging.handlers.RotatingFileHandler`.
+    * **Date** — when the local date changes, logging switches to a fresh
+      ``langbot-<new date>.log`` file. This happens even within a single
+      long-running process, so a bot started on day N keeps writing to that
+      day's file and rolls over to day N+1's file at midnight, instead of
+      appending every subsequent day's logs to the start-day file.
+
+    The on-disk naming stays compatible with the log-retention cleanup in
+    ``api/http/service/maintenance.py`` (``LOG_FILE_PATTERN``).
+    """
+
+    def __init__(self, log_dir: str, max_bytes: int, backup_count: int, encoding: str = 'utf-8'):
+        self.log_dir = log_dir
+        self._current_date = self._today()
+        super().__init__(
+            self._build_path(self._current_date),
+            maxBytes=max_bytes,
+            backupCount=backup_count,
+            encoding=encoding,
+        )
+
+    @staticmethod
+    def _today() -> str:
+        return time.strftime('%Y-%m-%d', time.localtime())
+
+    def _build_path(self, date_str: str) -> str:
+        return os.path.join(self.log_dir, 'langbot-%s.log' % date_str)
+
+    def shouldRollover(self, record):
+        # Roll over when the day changes, regardless of file size.
+        if self._today() != self._current_date:
+            return True
+        return super().shouldRollover(record)
+
+    def doRollover(self):
+        today = self._today()
+        if today != self._current_date:
+            # Date changed: point the handler at the new day's file.
+            # This is a date switch, not a size-based numbered rotation.
+            if self.stream:
+                self.stream.close()
+                self.stream = None
+            self._current_date = today
+            self.baseFilename = os.path.abspath(self._build_path(today))
+            if not self.delay:
+                self.stream = self._open()
+        else:
+            # Same day, file exceeded maxBytes: numbered rotation.
+            super().doRollover()
+
 
 async def init_logging(extra_handlers: list[logging.Handler] = None) -> logging.Logger:
     # Remove all existing loggers
@@ -31,8 +92,6 @@ async def init_logging(extra_handlers: list[logging.Handler] = None) -> logging.
     if constants.debug_mode:
         level = logging.DEBUG
 
-    log_file_name = 'data/logs/langbot-%s.log' % time.strftime('%Y-%m-%d', time.localtime())
-
     qcg_logger = logging.getLogger('langbot')
 
     qcg_logger.setLevel(level)
@@ -48,12 +107,13 @@ async def init_logging(extra_handlers: list[logging.Handler] = None) -> logging.
     # stream_handler.setFormatter(color_formatter)
     stream_handler.stream = open(sys.stdout.fileno(), mode='w', encoding='utf-8', buffering=1)
 
-    # Use RotatingFileHandler to prevent unbounded log file growth
-    rotating_file_handler = logging.handlers.RotatingFileHandler(
-        log_file_name,
+    # Rotate by size within a day and switch files when the date changes,
+    # so long-running processes still produce a log file for the current day.
+    rotating_file_handler = DailyGroupedRotatingFileHandler(
+        LOG_DIR,
+        max_bytes=LOG_FILE_MAX_BYTES,
+        backup_count=LOG_FILE_BACKUP_COUNT,
         encoding='utf-8',
-        maxBytes=LOG_FILE_MAX_BYTES,
-        backupCount=LOG_FILE_BACKUP_COUNT,
     )
 
     log_handlers: list[logging.Handler] = [