🔧 Add contract events for off-chain indexing

[Smartdevs17]

Problem

No events emitted for subscription changes.

Context

Off-chain systems cannot track contract activity efficiently.

Proposed Solution

Add comprehensive events:

1. PlanCreated event
2. SubscriptionStarted event
3. PaymentProcessed event
4. SubscriptionCancelled event
5. Include relevant data in events

Acceptance Criteria

• All actions emit events
• Events include indexed fields
• Topics for filtering
• Backward compatible additions

Technical Notes

File: contracts/src/lib.rs
Pattern: Soroban events

Constraints

• Gas costs
• Event size limits
• Indexing infrastructure

[drips-wave]

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