feat: add structured JSON logging using structlog (closes #19)

Assignment 12/api/logging_config.py

@@ -0,0 +1,23 @@
+import structlog
+import logging
+import sys
+
+def configure_logging():
+    """Configure structlog for JSON output to stdout."""
+
+    structlog.configure(
+        processors=[
+            structlog.stdlib.add_log_level,
+            structlog.stdlib.add_logger_name,
+            structlog.processors.TimeStamper(fmt="iso"),
+            structlog.processors.StackInfoRenderer(),
+            structlog.processors.format_exc_info,
+            structlog.processors.JSONRenderer(),
+        ],
+        wrapper_class=structlog.stdlib.BoundLogger,
+        context_class=dict,
+        logger_factory=structlog.PrintLoggerFactory(file=sys.stdout),
+    )
+
+def get_logger(name: str = __name__):
+    return structlog.get_logger(name)

Assignment 12/handlers/routes/transactions.py

@@ -38,6 +38,9 @@
     BusinessRuleViolationError,
     InvalidStateTransitionError,
 )
+from api.logging_config import get_logger
+
+logger = get_logger("sentinelpay.transactions")
 
 router = APIRouter(prefix="/api/transactions", tags=["Transactions"])
 
@@ -51,6 +54,7 @@ def _handle_service_errors(exc: Exception) -> HTTPException:
         return HTTPException(status_code=422, detail=str(exc))
     if isinstance(exc, InvalidStateTransitionError):
         return HTTPException(status_code=409, detail=str(exc))
+    logger.error("unhandled_error", error=str(exc), exc_info=True)
     return HTTPException(status_code=500, detail=f"Internal error: {exc}")
 
 
@@ -91,6 +95,12 @@ def submit_transaction(
             longitude=body.longitude,
             is_international=body.is_international,
         )
+        logger.info(
+            "transaction_submitted",
+            transaction_id=str(txn.transaction_id),
+            amount=body.amount,
+            currency=body.currency,
+        )
         # ── Increment metrics ──
         transactions_total.labels(channel=body.channel).inc()
 
@@ -182,6 +192,11 @@ def apply_decision(
             model_scores=[s.model_dump() for s in body.model_scores],
             account_tier=body.account_tier,
         )
+        logger.info(
+            "fraud_decision_applied",
+            transaction_id=transaction_id,
+            fraud_score=body.fraud_score,
+        )
         return TransactionResponse.from_domain(txn)
     except Exception as e:
         raise _handle_service_errors(e)