Feature/53 dispute case management

[mitchellecm7]

Closes #53

What this PR does
Adds backend endpoints to complete the dispute case management system.

New Endpoints

Method	Route	Purpose
POST	/api/disputes/[id]/resolve	Resolve dispute with resolution log
PATCH	/api/disputes/[id]/contract	Update contract state

Existing Endpoints (unchanged)

Method	Route	Purpose
POST	/api/disputes	Raise dispute
GET	/api/disputes	List disputes
GET	/api/disputes/[id]	Get dispute detail + evidence + comments
POST	/api/disputes/[id]/evidence	Submit evidence

Features

• ✅ Secure — all routes use withAuth middleware
• ✅ Validated — input validation with specific error codes
• ✅ Extensible — modular route design (DAO voting ready)
• ✅ Audit trail — resolution log tracks previous/new state + resolver
• ✅ Contract-aware — updates escrow status on jobs table

/claim #53

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