[TD-04] Add evidence attachment to maintenance records

[licette32]

Context

Point noted by Isaac Klassen (mentor, Milestone 1 feedback):

"it would be good to define what evidence is attached, and who approves it"

Currently, the hospital confirms that the maintenance was performed but without any verifiable attached evidence.

Plan

1. Add evidence_url field to the maintenance_events table in Supabase
2. The technician can upload a photo or PDF to Supabase Storage upon completing the job
3. Optional: calculate SHA256 of the file and save it as metadata in the DB to verify integrity
4. The on-chain Asset PDA does not need changes

UI

• In Technician Mode: "Attach evidence" button before calling complete_maintenance
• In Hospital Mode: view attached evidence before approving

Milestone: v1.1