Create a custom `logging.Filter` that automatically extracts the current OpenTelemetry trace context and injects `trace_id` and `span_id` into every log record

[psschwei]

Description:
Create a custom logging.Filter that automatically extracts the current OpenTelemetry trace context and injects trace_id and span_id into every log record. This enables correlation between logs and distributed traces.

Detailed Requirements:

1. Create OtelTraceFilter class (extends logging.Filter) that:
   • Gets current span from opentelemetry.trace.get_current_span()
   • Extracts trace_id and span_id from span context
   • Adds fields to LogRecord as attributes
   • Handles case when no active span (sets to None or empty string)
2. Add filter to FancyLogger's handlers
3. Update JsonFormatter to include trace_id/span_id fields
4. Update CustomFormatter to optionally show trace context
5. Ensure zero overhead when OpenTelemetry not installed (graceful degradation)
6. Format trace_id/span_id as hex strings (standard OTEL format)

Files to Modify:

• mellea/core/utils.py - Add OtelTraceFilter, update formatters

Acceptance Criteria:

• trace_id and span_id appear in all JSON log records when tracing is active
• Logs work correctly when tracing is disabled (fields are null/absent)
• No import errors when opentelemetry not installed
• Filter added to FancyLogger initialization