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/app.py

@@ -118,6 +118,14 @@ async def submit_transaction(body: TransactionRequest):
     try:
         result = await orchestrator.create_transaction(body.model_dump())
     except UpstreamFailure as exc:
+        log = logger.error if exc.status_code >= 500 else logger.warning
+        log(
+            "transaction_failed",
+            account_id=body.account_id,
+            status_code=exc.status_code,
+            reason=exc.message,
+            cause=repr(exc.__cause__) if exc.__cause__ else None,
+        )
         return JSONResponse(
             status_code=exc.status_code, content={"error": exc.message, "request_id": request_id}
         )