feat: implement transfer_subscription for ownership transition

[Abdullahi-Code9]

Closes #495

Summary

Implements transfer_subscription as described in issue #495 (CONTRACT-40).

Changes

contract/src/errors.rs

• Added SubscriptionAlreadyActive = 21 error variant
• Added MerchantFrozen = 22 error variant

contract/src/events.rs

• Added publish_subscription_transferred(env, old_user, new_user) emitting sub_transferred event

contract/src/lib.rs

• Implemented transfer_subscription(env, user, new_user) with:
  • Contract pause check
  • Auth requirement on original user
  • Validation that caller has an active subscription
  • Validation that new user does not already have an active subscription
  • Storage move from old key to new key
  • TTL refresh on new user's subscription
  • Event emission

contract/src/test.rs

• Added test_transfer_subscription_succeeds — verifies data is correctly moved to new user and removed from old user
• Added test_transfer_subscription_to_active_user_panics — verifies transfer to an already-active address panics

Test Result

124/124 tests passing, 0 failing

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