docs: add architecture diagrams to README and docs

[Sagargupta16]

Summary

• Added 4 Mermaid-generated SVG architecture diagrams (system overview, upload pipeline, auth flow, backend layers)
• Replaced ASCII box diagrams in docs/architecture.md with SVGs
• Added visual architecture section to README with embedded images
• Kept .mmd source files for future editing

Diagrams Added

• System Overview - deployment infrastructure and component connections
• Upload & Sync Pipeline - ingestion flow with reconciliation engine
• Authentication Flow - OAuth 2.0 sequence diagram
• Backend Layer Architecture - API > business logic > data access layers

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud