Fix study role expiration and DM

[pushiscool]

Added Task 4.

Root cause:

• /study stored the Study role expiration time in the database, but removal mostly depended on bot.loop.call_later(...)
• That timer is only in memory, so it can be lost if the bot restarts, crashes, or redeploys

Changes:

• Keeps /study limited to 10 minutes through exactly 1 week
• Allows 168h and blocks anything over 1 week
• Sends the user a DM with the expected Study role removal time
• Removes expired Study roles after restart using saved study_expires_at
• Handles missing Study role
• Includes CI fix for missing config role IDs