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>

services/api/src/logger.ts

@@ -0,0 +1,12 @@
+import pino from 'pino';
+import * as Sentry from '@sentry/node';
+
+if (process.env['SENTRY_DSN']) {
+  Sentry.init({
+    dsn: process.env['SENTRY_DSN'],
+    environment: process.env['NODE_ENV'] ?? 'development',
+  });
+}
+
+export const logger = pino({ level: process.env['LOG_LEVEL'] ?? 'info' });
+export { Sentry };