fix: Emit MilestoneRejected event in reject_milestone()

[0xdevcollins]

Summary

• reject_milestone() changed milestone status to Rejected but emitted no event, making rejections invisible to the backend indexer
• Adds MilestoneRejected event struct with campaign_id and milestone_id fields, matching the existing MilestoneApproved pattern
• Emits the event after status update in reject_milestone()

Closes #5

Test plan

• test_reject_milestone — existing test covers the rejection flow, now with event emission
• All 13 unit tests pass
• Build compiles clean

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 8e9bb99e-d5a4-47fc-84af-57ca674ab6cc

📥 Commits

Reviewing files that changed from the base of the PR and between 8e4abed and b9c7491.

📒 Files selected for processing (2)

• contracts/crowdfund_registry/src/contract.rs
• contracts/crowdfund_registry/src/events/mod.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/milestone-rejected-event

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