Is your feature request related to a problem? Please describe.
Currently, Speedtest Tracker allows configuring a single cron schedule for running speed tests. To provide more flexibility and better meet user needs, it would be beneficial to support multiple cron schedules. This feature would enable users to define different schedules for different times of the day, such as more frequent tests during peak hours and less frequent tests during off-peak hours.

Describe the solution you'd like
Allow users to define multiple cron expressions using a comma-separated list in the environment variable SPEEDTEST_SCHEDULE.

Additional context
Use Case Example:

• Frequent Testing During Peak Hours:
  Every 15 minutes from 6 AM to 10 PM.
  Cron expression: */15 6-22 * * *

• Hourly Testing During Off-Peak Hours:
  Once an hour from 11 PM to 5 AM.
  Cron expression: 0 23,0-5 * * *

These schedules would be combined into the SPEEDTEST_SCHEDULE variable as follows:

SPEEDTEST_SCHEDULE="*/15 6-22 * * *,0 23,0-5 * * *"