fix: resolve audit/maintenance issues

[Mkalbani]

Summary

Resolves issues #1473, #1474, #1475, and #1476 with minimal, efficient improvements to audit and maintenance systems.

Issues Fixed

#1473 - Unclear distinction between audit-log and audits dashboard pages

Problem: Overlapping page names with no explanation of differences.

Solution: Added clear documentation and cross-references to distinguish:

• Audit Log: System action tracking (create, update, delete) for compliance and accountability
• Audits / Stocktake: Physical asset verification to match system records

#1474 - Maintenance events don't record the technician's identity

Problem: MaintenanceCompleted events lacked technician identity, weakening on-chain audit trail.

Solution: Added technician_id: String field to MaintenanceCompleted event struct and updated the event emission to include technician data from the maintenance record.

#1475 - No upper bound check on logged maintenance cost

Problem: Maintenance costs weren't validated for upper limits, allowing extreme values.

Solution: Added validation check with max bound of 999_999_999_999 on labor_cost, parts_cost, and total_cost to prevent malformed or extreme numbers from skewing calculations.

#1476 - No test for unauthorized maintenance logging

Problem: No coverage for unauthorized callers attempting to log maintenance entries.

Solution: Added test_unauthorized_maintenance_logging() test that verifies an unauthorized provider address fails authentication when attempting to add a maintenance record.

Changes

• ✅ contracts/asset-maintenance/src/events.rs: Enhanced MaintenanceCompleted struct with technician tracking
• ✅ contracts/asset-maintenance/src/lib.rs: Added cost upper bound validation
• ✅ contracts/asset-maintenance/src/test.rs: Added unauthorized access test
• ✅ frontend/app/(dashboard)/audit-log/page.tsx: Added documentation and clarification text
• ✅ frontend/app/(dashboard)/audits/page.tsx: Added documentation and cross-reference

Testing

All changes are minimal and focused:

• Authorization check already exists; new test validates it rejects unauthorized callers
• Cost validation integrated into existing validation chain
• Event enhancement backward-compatible with existing event structure
• Frontend documentation only (no logic changes)

Closes #1473
Closes #1474
Closes #1475
Closes #1476

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