Requiring approval for subscriptions

**What**

Implement an approval workflow where subscription requests require owner approval before becoming active.

**Why**

Asset owners may want to vet subscribers before granting access. This enables gated access, membership review, or compliance requirements where not all subscription requests should be automatically granted.

**How**

- Add a pending subscriptions mechanism (e.g. mapping or struct for pending requests)
- Modify the subscribe flow to create a pending request instead of immediately activating
- Implement `approveSubscription(address)` and `rejectSubscription(address)` with onlyOwner modifier
- Ensure users can only have one pending request per asset
- Add tests for pending, approved, and rejected flows

**Acceptance Criteria**

- Subscription requests go into a pending state instead of immediate activation
- Asset owners can approve or reject pending subscriptions
- Approved subscriptions become active; rejected ones are cleared
- Users cannot bypass the approval flow
- Comprehensive test coverage including approval and rejection flows

**Estimation**

**Dependencies**

Depends on #9 (Implement Asset Contract)