feat(observability): structured logs, W3C trace IDs, Sentry hooks (#18)

- TS: pino + pino-http; every HTTP request log includes traceId from
  W3C traceparent header (generated if absent); forwarded to ml/serving
  on all /score, /generate, /reward, and /api/ml proxy calls
- Python: structlog JSON; FastAPI middleware binds trace_id via
  contextvars so every log line within a request carries it
- Sentry: optional SENTRY_DSN init in both runtimes (no-op if unset)
- Replace all console.* calls across services/api with pino logger
- Update tests to spy on logger instead of console

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ml/serving/logging_config.py

@@ -0,0 +1,20 @@
+"""Structlog JSON configuration — import once at process start."""
+import logging
+import structlog
+
+
+def configure() -> None:
+    structlog.configure(
+        processors=[
+            structlog.contextvars.merge_contextvars,
+            structlog.stdlib.add_log_level,
+            structlog.stdlib.add_logger_name,
+            structlog.processors.TimeStamper(fmt="iso"),
+            structlog.processors.StackInfoRenderer(),
+            structlog.processors.JSONRenderer(),
+        ],
+        wrapper_class=structlog.make_filtering_bound_logger(logging.INFO),
+        context_class=dict,
+        logger_factory=structlog.PrintLoggerFactory(),
+    )
+    logging.basicConfig(level=logging.WARNING)