feat: add milestone deadlines, time-based validation, and auto-expiry

[chisomvictorcv-sketch]

Closes #78

Summary
Enhances the escrow contract to support milestone deadlines, time-based validation, and an auto-expiry mechanism for overdue/incomplete milestones.

Changes

Soroban Contract (contracts/contracts/escrow/src/lib.rs)

• Added deadline: u64 field to Milestone struct
• Added AutoExpired status to MilestoneStatus enum
• Added DeadlineExceeded and AlreadyExpired error variants
• Added MilestoneDeadline(u32) data key for per-milestone deadline storage
• Deadlines stored during initialize()
• submit_milestone() validates against deadline — rejects if expired
• New auto_expire() function — anyone can trigger expiry after deadline, refunds client
• New is_milestone_expired() query function
• New get_milestone_deadline() query function

TypeScript Types (lib/escrow/types.ts)

• Added 'auto_expired' to MilestoneStatus union
• Added deadline?: number (Unix timestamp) to MilestoneInput

Acceptance Criteria

• Support multiple milestones within a contract (already existed)
• Allow partial fund releases per milestone (already existed via release())
• Validate milestones based on time constraints
• Auto-expiry option for overdue or incomplete milestones

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