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/gateway/orchestrator.py

@@ -1,8 +1,8 @@
-import logging
 import time
 import uuid
 
 import httpx
+import structlog
 from opentelemetry import propagate, trace
 from opentelemetry.trace import SpanKind
 
@@ -12,7 +12,7 @@ from common.settings import settings
 
 from . import chaos
 
-logger = logging.getLogger("gateway")
+logger = structlog.get_logger("gateway")
 tracer = trace.get_tracer(__name__)
 
 REJECT_THRESHOLD = 0.8
@@ -76,7 +76,12 @@ async def _run_chatty_health_probe(client: httpx.AsyncClient) -> None:
 
 async def create_transaction(payload: dict) -> dict:
     start = time.perf_counter()
-    logger.info(f"start transaction for account {payload['account_id']}")
+    logger.info(
+        "transaction_started",
+        account_id=payload["account_id"],
+        kind=payload["kind"],
+        amount_minor=payload["amount_minor"],
+    )
 
     retry_storm = await chaos.retry_storm_enabled()
     no_batching = await chaos.no_batching_enabled()
@@ -155,7 +160,7 @@ async def create_transaction(payload: dict) -> dict:
                 msg_id = await redis_client.xadd(settings.stream_name, fields)
                 span.set_attribute("messaging.message.id", msg_id)
         except Exception:
-            logger.exception(f"failed to publish transaction {tx_id} to the stream")
+            logger.exception("stream_publish_failed", transaction_id=tx_id)
 
         summary = {}
         try:
@@ -165,10 +170,16 @@ async def create_transaction(payload: dict) -> dict:
             if summary_resp.status_code == 200:
                 summary = summary_resp.json()
         except httpx.RequestError:
-            logger.exception(f"insights summary unavailable for transaction {tx_id}")
+            logger.exception("insights_summary_unavailable", transaction_id=tx_id)
 
     elapsed_ms = (time.perf_counter() - start) * 1000
-    logger.info(f"finished transaction {tx_id} status={settled['status']}")
+    logger.info(
+        "transaction_finished",
+        transaction_id=tx_id,
+        status=settled["status"],
+        risk_score=risk_score,
+        duration_ms=round(elapsed_ms, 1),
+    )
 
     return {
         "transaction_id": tx_id,