Add overdue milestone enforcement beyond event emission

[0xdevcollins]

Problem

flag_overdue_milestone() only emits a MilestoneOverdue event but performs no state transition or enforcement. Overdue milestones have no on-chain consequence.

Current Behavior

• flag_overdue_milestone() checks 30-day overdue threshold
• Emits MilestoneOverdue event
• No state change, no refund trigger, no escalation

Expected Behavior

Add enforcement options after flagging:

1. Option A — Auto-escalate: After flagging, if no submission within a grace period (e.g., 14 more days), allow anyone to call escalate_overdue_milestone() which:

   • Transitions milestone to Rejected
   • Optionally triggers campaign failure if critical milestone

2. Option B — Admin action required: Flag sets a flagged_overdue: true field, admin must then approve extension or reject milestone

Acceptance Criteria

• Overdue milestones have an enforceable consequence
• Grace period mechanism to give creators time to respond
• Events emitted for backend indexer
• Tests cover overdue → escalation → resolution flow

Priority

Medium — Currently relies entirely on off-chain monitoring with no teeth.

Labels

crowdfunding, smart-contract, enhancement