fix(scheduler): honor app timezone for cron triggers and log scheduled events

CronTrigger.from_crontab was constructed without a timezone, so a cron like
'0 9 * * *' fired at 09:00 host-local instead of 09:00 in the admin-configured
timezone. Now all tracker/action cron triggers are built with the app tz, and
the setting endpoint rebuilds existing cron jobs when the tz changes (since
CronTrigger freezes its tz at construction time).

The scheduler provider also renders current_date/time/datetime/weekday in the
configured tz and exposes a new 'timezone' template variable.

EventLog entries for scheduled_message now include schedule_type,
cron_expression/interval_seconds, timezone, and fire_count, and the dashboard
shows the event type with a label/icon/color.

packages/server/src/notify_bridge_server/api/template_configs.py

@@ -242,6 +242,8 @@ async def get_template_variables(
                 "current_date": "Current date (formatted)",
                 "current_time": "Current time (formatted)",
                 "current_datetime": "Current date and time (formatted)",
+                "weekday": "Day of the week (Monday..Sunday)",
+                "timezone": "IANA timezone used for current_date/time",
             },
         },
     }