Add disable_on_expiry flag for service subscriptions

[ValtteriTuni]

closes #597

What is in this PR:

1. User clicks "Unsubscribe service"
   -flag is set to True
   -Subscription remains active
   -Message shows: "Service will be unsubscribed on [date] when your paid days run out"
   -Button changes to "Cancel unsubscribe"

2. User clicks "Cancel unsubscribe"
   -flag is set to false
   -Subscription continues normally
   -Button changes back to "Unsubscribe service"

3. Subscription expires with disable_on_expiry = true:
   -Subscription is deleted
   -User access ends on exact paid_until date
   -Action is logged

4. Subscription expires with disable_on_expiry = false (default, unchanged):
   -Subscription goes to overdue state
   -User gets reminders

5. User makes payment while disable_on_expiry = True
   -Flag is cleared
   -Paid days are aded normally
   -Subscription continues - unsubscribe is cancelled

6. Admin panel

• New column shows disable_on_expiry status
• Filterable by disable_on_expiry
• editable
• Visible in ServiceSubscription admin

7. User profile page
   -Badge shows "Will auto-unsubscribe on expiry" when flag is set
   -User settings shows "Pending unsubscribe"

8. 3 Test cases