Implement `raised_amount` update and milestone unlock check

[ayshadogo]

User Story:
As a developer, I want the contract to automatically detect when milestones are reached after each donation so that unlocking happens on-chain.
Acceptance Criteria:

• After donation, raised_amount updated atomically
• Loop over milestones: if raised_amount >= milestone.target_amount && status == Locked, set to Unlocked
• milestone_unlocked event emitted for each newly unlocked milestone
• Multiple milestones can unlock in single donation

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