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/ledger/app.py

@@ -52,7 +52,7 @@ class SettleRequest(BaseModel):
 
 @app.exception_handler(Exception)
 async def unhandled_exception_handler(request: Request, exc: Exception):
-    logger.exception(f"unhandled exception on {request.url.path}: {exc}")
+    logger.exception("unhandled_exception", path=request.url.path)
     return JSONResponse(status_code=500, content={"error": "internal error"})
 
 
@@ -64,7 +64,7 @@ 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}
 
 
@@ -77,7 +77,7 @@ 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"}
 
@@ -91,7 +91,12 @@ async def list_accounts():
 
 @app.post("/internal/transactions")
 async def create_transaction(body: CreateTransactionRequest):
-    logger.info(f"processing tx for account {body.account_id}, amount {body.amount_minor}")
+    logger.info(
+        "transaction_received",
+        account_id=body.account_id,
+        kind=body.kind,
+        amount_minor=body.amount_minor,
+    )
 
     starved = await chaos.pool_starvation_enabled()
     poison_enabled = await chaos.poison_data_enabled()
@@ -151,7 +156,7 @@ async def settle(tx_id: str, body: SettleRequest):
             use_lock = row_lock and bool(account.is_hot)
             balance = await repository.settle_transaction(session, tx=tx, risk_score=body.risk_score, use_row_lock=use_lock)
 
-    logger.info(f"settled transaction {tx_id}")
+    logger.info("transaction_settled", transaction_id=tx_id, risk_score=body.risk_score)
     return {"status": "settled", "balance_minor": balance}
 
 
@@ -165,5 +170,5 @@ async def reject(tx_id: str, body: SettleRequest):
             return JSONResponse(status_code=404, content={"error": "transaction not found"})
         balance = await repository.reject_transaction(session, tx=tx, risk_score=body.risk_score)
 
-    logger.info(f"rejected transaction {tx_id}")
+    logger.info("transaction_rejected", transaction_id=tx_id, risk_score=body.risk_score)
     return {"status": "rejected", "balance_minor": balance}