[Analytics] Add error rate metrics endpoint

[PierreLeGuen]

Description

Add analytics for tracking error rates across the platform, by model, and by organization.

Motivation

Understanding error patterns helps identify problematic models, infrastructure issues, or organizations experiencing issues.

Proposed Solution

Add error metrics to existing endpoints or create new endpoint:

• Error count and rate (errors/total requests)
• Breakdown by error type (rate_limit, inference_error, invalid_params, etc.)
• Breakdown by model
• Breakdown by organization (top orgs with errors)

Technical Details

• Track errors in organization_usage_log or separate error log table
• Include error_type field for categorization
• Support time range filtering

Acceptance Criteria

• Error rate metrics available via API
• Breakdown by error type
• Breakdown by model
• Time series support for error trends