feat: add lightweight request tracing for debugging

Adds JSON-based tracing system for local development that captures
the full request flow through Tatlock's multi-agent architecture.

- Trace/Span dataclasses with automatic timing and nesting
- Context-var based propagation for async-safe tracing
- trace_span async context manager for clean instrumentation
- Traces written to logs/traces/{trace_id}.json
- REST API for listing and retrieving traces (/traces)
- Standalone HTML viewer with timeline visualization

Enabled via DEBUG=true environment variable.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -68,7 +68,10 @@ dmypy.json
 .ruff_cache/
 
 # Logs
-logs/
+logs/*
+!logs/traces/
+logs/traces/*
+!logs/traces/viewer.html
 *.log
 
 # Database