Rewrite every log call as an event name with typed fields

Prose f-strings become past-tense events with consistent field names
across services (account_id, transaction_id, amount_minor, risk_score,
status_code, consumer_group, message_id, chaos_flag). The orchestrator
and both consumers move from logging.getLogger to structlog so they can
pass fields.

New events cover what was silent before: the gateway's UpstreamFailure
path (transaction_failed, with the underlying cause), fraud's score and
sanctions' screening outcomes, and each successfully processed stream
message. holder_name is no longer logged at all.

"lookup" and "healthz" still appear in exactly one event each, so the
pathology tests' log-line counts are unchanged until they move to LogQL.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

services/insights/app.py

@@ -59,7 +59,7 @@ async def root():
 @app.get("/healthz")
 async def healthz(request: Request):
     if request.headers.get("X-Ledgerline-Probe") == "chatty-health":
-        logger.info("received chatty-health healthz probe from gateway")
+        logger.info("healthz_probe_received", probe="chatty-health")
     return {"status": "ok", "service": SERVICE_NAME}
 
 
@@ -72,7 +72,7 @@ async def readyz():
         span = trace.get_current_span()
         span.record_exception(exc)
         span.set_status(StatusCode.ERROR, str(exc))
-        logger.exception(f"readyz check failed: {exc}")
+        logger.exception("readyz_check_failed")
         return JSONResponse(status_code=503, content={"status": "unavailable"})
     return {"status": "ok"}