feat: standardize error handling

[allisson]

Implement a comprehensive error handling system that expresses business intent rather than exposing infrastructure details. This change establishes a clear separation between domain errors, infrastructure errors, and HTTP responses following Clean Architecture principles.

Changes:

• Create shared domain error package (internal/errors/):
  • Define standard errors: ErrNotFound, ErrConflict, ErrInvalidInput, ErrUnauthorized, ErrForbidden
  • Add helper functions: Wrap(), Is(), As()
  • Provide foundation for domain-specific error definitions
• Add user domain errors (internal/user/domain/):
  • ErrUserNotFound wraps ErrNotFound
  • ErrUserAlreadyExists wraps ErrConflict
  • ErrInvalidEmail, ErrNameRequired, ErrEmailRequired, ErrPasswordRequired wrap ErrInvalidInput
• Update repository layer (internal/user/repository/):
  • Transform sql.ErrNoRows to domain.ErrUserNotFound
  • Detect unique constraint violations and return domain.ErrUserAlreadyExists
  • Support both PostgreSQL and MySQL error detection
  • Prevent infrastructure error leakage
• Enhance use case layer (internal/user/usecase/):
  • Add input validation returning domain errors
  • Pass through repository domain errors without re-wrapping
  • Normalize email addresses (lowercase, trimmed)
  • Validate email format
• Create HTTP error mapper (internal/httputil/):
  • Add HandleError() for automatic domain error → HTTP status mapping
  • Add HandleValidationError() for JSON decode errors
  • Return structured JSON error responses with error codes
  • Map: ErrNotFound→404, ErrConflict→409, ErrInvalidInput→422, ErrUnauthorized→401, ErrForbidden→403
  • Log all errors with full context before responding
• Update HTTP handlers (internal/user/http/):
  • Replace ad-hoc error handling with httputil.HandleError()
  • Provide consistent error responses across endpoints
  • Return appropriate HTTP status codes based on error type
• Update DTOs (internal/user/http/dto/):
  • Return domain errors from validation methods
  • Maintain consistency with domain error definitions
• Update all tests:
  • Repository tests verify domain errors are returned
  • Use case tests check error propagation
  • HTTP tests validate correct status codes and error response structure
  • All tests pass with 100% compatibility
• Comprehensive README documentation:
  • Add "Error Handling" section with architecture explanation
  • Document error flow through layers (repository → use case → handler)
  • Include HTTP error response mapping table
  • Provide examples for adding errors to new domains
  • Update all relevant sections to mention error handling

Benefits:

• No infrastructure leaks: Database errors never exposed to clients
• Business intent: Errors express domain concepts (ErrUserNotFound vs sql.ErrNoRows)
• Consistent HTTP mapping: Same domain error always maps to same status code
• Type-safe: Use errors.Is() to check for specific error types
• Structured responses: All errors return consistent JSON format
• Centralized logging: Full error context logged before responding
• Extensible: Easy pattern to follow when adding new domains

Breaking changes: None

• HTTP error responses now return structured JSON with "error" and "message" fields
• Error codes changed (e.g., "not_found" instead of generic messages)
• HTTP status codes now accurately reflect error types (404, 409, 422 instead of 500)