API consistency pass: move compare/upload data reads fully to backend APIs

[ApexYash11]

Summary

Frontend compare/upload flows should not rely on direct DB table reads or custom parsing paths.

Why

Current architecture docs identify data consistency risk when business data is fetched from multiple pathways.

Scope

• Audit compare and upload frontend paths for direct table access.
• Route all business reads through backend API endpoints.
• Remove duplicate client-side parsing logic now replaced by backend contracts.
• Ensure response shapes are typed and consistent in frontend adapters.

Acceptance Criteria

• Compare and upload flows use backend APIs as single source of truth.
• No direct frontend business reads from DB tables in these flows.
• Existing UX behavior remains intact with clear error handling.

Source

• docs/SITE_FLOW_AND_IMPROVEMENTS.md (Improvement Plan V2 #1, Success Criteria)