feat: Python SDK real implementation + API ingestion routes
- SDK: client with BatchTransport, trace decorator/context manager, log_decision, thread-local context stack, nested trace→span support - API: POST /api/traces (batch ingest), GET /api/traces (paginated list), GET /api/traces/[id] (full trace with relations), GET /api/health - Tests: 8 unit tests for SDK (all passing) - Transport: thread-safe buffer with background flush thread
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