IAV-691/feat(common): Add error codes prefix registry for microservices

Summary

Adds a centralized error codes prefix registry to iavia-common to ensure unique and consistent error codes across all microservices.

Changes

• Added error codes prefix registry documentation
• Defined standard format: [PREFIX]-[NUM]
• Mapped prefixes for all agent services (community-manager, customer-care, legal, hr, seo)
• Mapped prefixes for all shared services (core-n8n, credit, doc-analysis, file-manager, orchestration, prompt-processing, user)

Why

This registry prevents error code collisions between microservices and provides a single source of truth for prefix assignments.

Closes IAV-691