Capping subscription count

**What**

Implement a configurable cap on the maximum number of active subscriptions per asset.

**Why**

Asset owners may want to limit how many users can subscribe to an asset at any given time. This supports scarcity, quality control, and capacity planning for the asset.

**How**

- Add a subscription cap (max subscriptions) to the Asset contract, configurable by owner
- Track the current number of active subscriptions
- Add a check in the subscribe flow to reject new subscriptions when the cap is reached
- Implement `setSubscriptionCap(uint256)` (or similar) with onlyOwner modifier
- Consider whether expired subscriptions count toward the cap or not
- Add tests for cap enforcement and owner configuration

**Acceptance Criteria**

- Asset owners can set a maximum subscription count per asset
- New subscriptions are rejected when the cap is reached
- Cap can be updated by the owner
- Behavior is well-defined for subscription expiry and cap
- Comprehensive test coverage

**Estimation**

**Dependencies**

Depends on #9 (Implement Asset Contract)